@article{copes_ojiambo_2023, title={A Systematic Review and Quantitative Synthesis of the Efficacy of Quaternary Ammonium Compounds in Disinfesting Nonfungal Plant Pathogens}, volume={3}, ISSN={["1943-7692"]}, DOI={10.1094/PDIS-12-21-2751-RE}, abstractNote={ This quantitative review and systematic analysis of the effectiveness of quaternary ammonium compounds (QACs) in disinfesting nonfungal plant pathogens in agricultural and horticultural cropping systems is a complementary follow-up to a previous study that evaluated the efficacy of QACs against fungal plant pathogens. In the present study, a meta-analysis involving 67 studies was conducted to assess the overall efficacy of QACs against plant pathogenic bacteria, oomycetes, and viruses and to identify factors associated with observed differences in product efficacy. Across all studies, QACs resulted in a significant (P < 0.0001) reduction in either disease intensity or propagule viability with a mean Hedges’ g ([Formula: see text]) of 1.75, indicating that overall QAC treatments were moderately effective against nonfungal pathogens. Significant differences in product efficacy were observed between organism types (P = 0.0001), with QAC interventions resulting in higher efficacy (P = 0.0002) against oomycetes ([Formula: see text] = 4.20) than against viruses ([Formula: see text] = 1.42) and bacteria ([Formula: see text] = 1.07), which were not different (P = 0.2689) from each other. As a result, bacterium and virus types were combined into a composite set (BacVir). QAC intervention against BacVir resulted in significant differences in efficacy within categorical moderator subgroups for genus (P = 0.0133), target material (P = 0.0001), and QAC product generation (P = 0.0281). QAC intervention against oomycetes resulted in significant differences in efficacy only for genus (P < 0.0001). For the BacVir composite, five random effect (RE) meta-regression models were significant (P = 0.05), where models with dose and time, dose and genus, time and genus, dose and target, and time and target accounted for 62, 61, 52, 83, and 88%, respectively, of the variance in true effect sizes (R2) associated with [Formula: see text]. For oomycetes, three RE meta-regression models were significant (P = 0.05), where models with dose and time, dose and genus, and time and genus accounted for 64, 86, and 90%, respectively, of R2 associated with [Formula: see text]. These results show that while QACs are moderately effective against nonfungal plant pathogens, the observed variability in their efficacy due to dose of active ingredient and contact time of these products can be influenced by organism type, genus within organism type, the target being treated, and the generation of QAC products. }, journal={PLANT DISEASE}, author={Copes, Warren E. and Ojiambo, Peter S.}, year={2023}, month={Mar} }
 @article{copes_ojiambo_2023, title={A Systematic Review and Quantitative Synthesis of the Efficacy of Quaternary Ammonium Disinfestants Against Fungal Plant Pathogens}, volume={2}, ISSN={["1943-7692"]}, DOI={10.1094/PDIS-09-21-2018-RE}, abstractNote={ Quaternary ammonium compounds (QACs) have been used as disinfestants in plant production systems since the late 20th century. In studies on the control of fungal pathogens in agricultural and horticultural crop production systems, the efficacy of QAC disinfestants is variable, ranging from very high to ineffective. A systematic review and meta-analysis were performed to establish and understand how pathogen- and application-related factors influenced product efficacy. The meta-analysis was based on 124 studies involving 14 fungal plant pathogen genera, eight target materials, and four generations of QAC products that contained different mixtures of active ingredients. A significant (P < 0.0001) reduction in either disease intensity or propagule viability resulted following disinfestation using QAC products. Hedges’ g standardized mean difference (g̅+) across the studies was 2.16, indicating that QACs, on average, were highly effective against fungal pathogens. Heterogeneity was significant (P < 0.0001), indicating that effect sizes (g) were not representative of a common mean effect size and supported selection of a random effects model. In all, 78.5% of the observed variance consisted of variance in true effects with a high estimate of between-study variability (τ2 = 2.15). For fungal genus, subgroup g̅+ for genera Pseudonectria and Calonectria was significantly (P < 0.0038) higher than for all other genus subgroups, except Fusarium. For target materials, subgroup g̅+ for solution, cloth, plant, and metal were significantly (P > 0.0071) higher than for inorganic material or wood. For product generation, subgroup g̅+ for fifth-generation products was significantly (P > 0.0071) higher than for fourth-, third-, and second-generation products. Dose and time accounted for only 8 and 4%, respectively, of the true variance in effect sizes in the regression model dose, time, and dose–time (P = 0.0004). Genus accounted for 40 and 51% of the true variance in effect sizes in the regression models dose and genus (P = 0.0008) and time and genus (P = 0.0007), respectively. Target material accounted for 18 and 19% of the true variance in effect sizes in the regression models dose and target (P = 0.0001) and time and target (P = 0.0001), respectively. QAC product generation accounted for 24 and 21% of the true variance in effect sizes in the regression models dose and QAC generation (P = 0.0034) and time and QAC generation (P = 0.0189), respectively. These results show that the current recommended rates for dose and contact time are generally expected to result in effective disinfestation for commercial QAC products. However, the efficacy against fungal plant pathogens is likely to be influenced by the fungal genus and target being treated and the generation of the QAC product that is used for disinfestation. }, journal={PLANT DISEASE}, author={Copes, Warren. E. E. and Ojiambo, Peter. S. S.}, year={2023}, month={Feb} }
 @article{adhikari_brown_ojiambo_cowger_2023, title={Effects of Host and Weather Factors on the Growth Rate of Septoria nodorum Blotch Lesions on Winter Wheat}, volume={5}, ISSN={["1943-7684"]}, url={https://doi.org/10.1094/PHYTO-12-22-0476-R}, DOI={10.1094/PHYTO-12-22-0476-R}, abstractNote={ Septoria nodorum blotch (SNB), caused by Parastagonospora nodorum, is a major disease of winter wheat that occurs frequently in the central and southeastern United States. Quantitative resistance to SNB in wheat is determined by various disease resistance components and their interaction with environmental factors. A study was conducted in North Carolina from 2018 to 2020 to characterize SNB lesion size and growth rate and to quantify the effects of temperature and relative humidity on lesion expansion in winter wheat cultivars with different levels of resistance. Disease was initiated in the field by spreading P. nodorum-infected wheat straw in experimental plots. Cohorts (groups of foliar lesions arbitrarily selected and tagged as an observational unit) were sequentially selected and monitored throughout each season. Lesion area was measured at regular intervals, and weather data were collected using in-field data loggers and the nearest weather stations. Final mean lesion area was approximately seven times greater on susceptible than on moderately resistant cultivars, and lesion growth rate was approximately four times higher on susceptible than on moderately resistant cultivars. Across trials and cultivars, temperature had a strong effect of increasing lesion growth rates ( P < 0.001), while relative humidity had no significant effect ( P = 0.34). Lesion growth rate declined slightly and steadily over the duration of cohort assessment. Our results demonstrate that restricting lesion growth is an important component of SNB resistance in the field and suggest that the ability to limit lesion size may be a useful breeding goal. }, journal={PHYTOPATHOLOGY}, author={Adhikari, Urmila and Brown, James and Ojiambo, Peter S. and Cowger, Christina}, year={2023}, month={May} }
 @misc{copes_ojiambo_2023, title={Efficacy of peroxygen disinfestants against fungal plant pathogens. A systemic review and meta-analysis}, volume={164}, ISSN={["1873-6904"]}, DOI={10.1016/j.cropro.2022.106143}, abstractNote={The peroxygen compounds, i.e. hydrogen peroxide, peracetic acid and potassium peroxymonosulfate, have been used as disinfestants in agricultural and horticultural operations for about 30 years. This systematic review was conducted to establish the overall efficacy of peroxygen compounds against fungal plant pathogens under production settings. A meta-analysis was performed to evaluate the biocidal activity of peroxygen intervention treatments compared to non-treated controls against 20 fungal genera of plant pathogens in 95 studies. The overall summary effect was a high Hedges' g value of 3.48 with 95% confidence limits of 3.02–3.93 (P < 0.0001) for the random effects model. This observation indicated that use of peroxygen compounds, in most cases, resulted in a high reduction in viable propagules or disease progression. However, heterogeneity was also high with 88.9% of the total variance accounted for by true variance and a high between-study variance of 3.71. To understand what influences heterogeneity, subgroup analyses were performed on the categorical moderators, i.e. fungal genera, target materials and peroxygen active ingredients (a.i.). In addition, two-variable meta-regression analyses were performed with the continuous moderators of peroxygen dose and/or contact time and the three categorical moderators. Subgroup analyses showed differences between target materials (P = 0.0151) and peroxygen a.i. (P = 0.0101) but not between fungal genera (P = 0.1753). Meta-regression results concurred with subgroup analysis results wherein models with target materials and dose (P = 0.0119) or time (P = 0.0122) accounted for 8 and 9% of the true variance, respectively. Models with peroxygen a.i. and dose (P = 0.0067) or time (P = 0.0093) accounted for 5 and 4% of the true variance, respectively. Thus, heterogeneity was only partly explained by the moderators evaluated and a larger portion of the true variance attributed to factors not available through the systematic review. Additional factors were evident also, such as diversity of research protocol, assessment measurements, sample size and small-study bias. The results support that the current doses and contact times recommended for peroxygen compounds will generally be effective at controlling fungal plant pathogens in agricultural and horticultural production systems. Results also indicate that efficacy of peroxygen compounds against fungi can be affected by the target material being treated and the peroxygen a.i. applied and potentially by fungal genus. This analysis serves as a base reference for considering efficacy performance of peroxygen compounds in these production settings.}, journal={CROP PROTECTION}, author={Copes, Warren E. and Ojiambo, Peter S.}, year={2023}, month={Feb} }
 @article{adhikari_cowger_ojiambo_2023, title={Evaluation of a Model for Predicting Onset of Septoria nodorum Blotch in Winter Wheat}, volume={107}, ISSN={["1943-7692"]}, url={https://doi.org/10.1094/PDIS-06-22-1469-RE}, DOI={10.1094/PDIS-06-22-1469-RE}, abstractNote={ Prediction models that aid growers in making decisions on timing of fungicide application are important components of integrated management programs for several foliar diseases of wheat. The risk of Septoria nodorum blotch (caused by Parastagonospora nodorum) onset in winter wheat has been reported to be influenced by location, amount of wheat residue in the field, and cumulative daily infection values 2 weeks prior to day of year (DOY) 102. A model previously developed based on these predictor variables was evaluated for its ability to predict disease onset under field conditions. An experiment was conducted at three locations in North Carolina in 2018, 2019, and 2020, where plots were either treated with >20% wheat residue or received no residue treatment. Plots were monitored for disease symptoms, and disease onset was defined to have occurred when mean disease incidence in a plot was 50%. Of the 298 disease cases recorded, disease onset occurred early (i.e., prior to DOY 102) in 257 cases, while onset was late (i.e., on or after DOY 102) in 41 cases. Model accuracy based on correct classification ranged from 0.67 to 0.95, with a mean of 0.87 across the study period. Similarly, sensitivity rates of the model ranged from 0.88 to 1.0 with a mean of 0.98 across all years. However, the model had low specificity, with a mean rate of 0.15 across the study period. Overall, there was no significant difference in the frequency of observed and predicted cases in the study (χ2 = 0.50, P = 0.7788, df = 2). Time to disease onset was significantly correlated with grain yield and explained 26% of variation in yield (P < 0.0001). Results indicated that the disease onset model performs well in predicting early disease onset but requires further evaluation and improvement, particularly in the Piedmont, where it over-predicted early onset in 2 successive years. }, number={4}, journal={PLANT DISEASE}, author={Adhikari, Urmila and Cowger, Christina and Ojiambo, Peter S.}, year={2023}, month={Apr}, pages={1122–1130} }
 @article{kikway_keinath_ojiambo_2023, title={Within-Season Shift in Fungicide Sensitivity Profiles of Pseudoperonospora cubensis Populations in Response to Chemical Control}, volume={4}, ISSN={["1943-7692"]}, DOI={10.1094/PDIS-09-22-2056-RE}, abstractNote={ Cucurbit downy mildew, caused by Pseudoperonospora cubensis, is an important disease affecting cucurbits worldwide. Chemical control is an effective method for disease control but P. cubensis has a high risk for developing resistance to fungicides. Alternating fungicides with different modes of action is recommended to avoid an increase of resistant subpopulations. Thus, this study was conducted to establish shifts in the sensitivity profiles of P. cubensis isolates during the growing season, wherein chlorothalonil was applied in alternation with either cymoxanil, fluopicolide, or propamocarb in field experiments conducted from 2018 to 2020 at Rocky Mount, NC and in 2018 and 2020 at Charleston, SC. The sensitivity of baseline isolates sampled early in the season or exposed isolates sampled late in the season to these single-site fungicides was determined using a detached-leaf assay, where tested isolates were classified as sensitive or resistant based on the relative disease severity. Based on the Kruskal-Wallis test, the distribution profile of relative disease severity among baseline and exposed isolates was significantly different where chlorothalonil was alternated with fluopicolide (χ2 = 10.82; P = 0.001) but not with cymoxanil (χ2 = 1.39; P = 0.238) or propamocarb (χ2 = 2.37; P = 0.412). Although there was a directional selection toward resistance for isolates sampled from plots that were treated with fluopicolide or propamocarb alternated with chlorothalonil during a growing season, a significant shift in fungicide sensitivity distribution based on combined data were observed for fluopicolide (χ2 = 8.25; P = 0.004) but not propamocarb (χ2 = 1.05; P = 0.461). Baseline and exposed isolates sampled from the cymoxanil-treated plots were all resistant to this fungicide and there was no significant shift in their fungicide sensitivity profile during a growing season (χ2 = 0.06; P = 1.000). These results indicate that a shift toward reduced sensitivity in P. cubensis can occur during a growing season and the efficacy of fluopicolide is likely to decrease as the frequency of the less sensitive subpopulations increases during a production season. The resultant effect on disease severity and selection of an insensitive subpopulation may accelerate the development of resistance to propamocarb in the southeastern United States. }, journal={PLANT DISEASE}, author={Kikway, Isaack and Keinath, Anthony P. and Ojiambo, Peter S.}, year={2023}, month={Apr} }
 @article{luis_carbone_mack_lebar_cary_gilbert_bhatnagar_wientjes_payne_moore_et al._2022, title={Dataset for transcriptomic profiles associated with development of sexual structures in Aspergillus flavus}, volume={42}, ISSN={["2352-3409"]}, DOI={10.1016/j.dib.2022.108033}, abstractNote={Information on the transcriptomic changes that occur within sclerotia of Aspergillus flavus during its sexual cycle is very limited and warrants further research. The findings will broaden our knowledge of the biology of A. flavus and can provide valuable insights in the development or deployment of non-toxigenic strains as biocontrol agents against aflatoxigenic strains. This article presents transcriptomic datasets included in our research article entitled, "Development of sexual structures influences metabolomic and transcriptomic profiles in Aspergillus flavus" [1], which utilized transcriptomics to identify possible genes and gene clusters associated with sexual reproduction and fertilization in A. flavus. RNA was extracted from sclerotia of a high fertility cross (Hi-Fert-Mated), a low fertility cross (Lo-Fert-Mated), and unmated strains (Hi-Fert-Unmated and Lo-Fert-Unmated) of A. flavus collected immediately after crossing and at every two weeks until eight weeks of incubation on mixed cereal agar at 30 °C in continuous darkness (n = 4 replicates from each treatment for each time point; 80 total). Raw sequencing reads obtained on an Illumina NovaSeq 6000 were deposited in NCBI's Sequence Read Archive (SRA) repository under BioProject accession number PRJNA789260. Reads were mapped to the A. flavus NRRL 3357 genome (assembly JCVI-afl1-v2.0; GCA_000006275.2) using STAR software. Differential gene expression analyses, functional analyses, and weighted gene co-expression network analysis were performed using DESeq2 R packages. The raw and analyzed data presented in this article could be reused for comparisons with other datasets to obtain transcriptional differences among strains of A. flavus or closely related species. The data can also be used for further investigation of the molecular basis of different processes involved in sexual reproduction and sclerotia fertility in A. flavus.}, journal={DATA IN BRIEF}, author={Luis, Jane Marian and Carbone, Ignazio and Mack, Brian M. and Lebar, Matthew D. and Cary, Jeffrey W. and Gilbert, Matthew K. and Bhatnagar, Deepak and Wientjes, Carol-Carter and Payne, Gary A. and Moore, Geromy G. and et al.}, year={2022}, month={Jun} }
 @article{luis_carbone_mack_lebar_cary_gilbert_bhatnagar_wientjes_payne_moore_et al._2022, title={Development of sexual structures influences metabolomic and transcriptomic profiles in Aspergillus flavus}, volume={126}, ISSN={["1878-6162"]}, DOI={10.1016/j.funbio.2022.01.001}, abstractNote={Sclerotium (female) fertility, the ability of a strain to produce ascocarps, influences internal morphological changes during sexual reproduction in Aspergillus flavus. Although sclerotial morphogenesis has been linked to secondary metabolite (SM) biosynthesis, metabolic and transcriptomic changes within A. flavus sclerotia during sexual development are not known. Successful mating between compatible strains may result in relatively high or low numbers of ascocarps being produced. Sclerotia from a high fertility cross (Hi-Fert-Mated), a low fertility cross (Lo-Fert-Mated), unmated strains (Hi-Fert-Unmated and Lo-Fert-Unmated) were harvested immediately after crosses were made and every two weeks until 8 weeks of incubation, then subjected to targeted metabolomics (n = 106) and transcriptomics analyses (n = 80). Aflatoxin B1 production varied between Hi-Fert-Mated and Hi-Fert-Unmated sclerotia, while it remained low or was undetected in Lo-Fert-Mated and Lo-Fert-Unmated sclerotia. Profiling of 14 SMs showed elevated production of an aflavazole analog, an aflavinine isomer, and hydroxyaflavinine in Hi-Fert-Mated sclerotia at 4 to 8 weeks. Similarly, genes ayg1, hxtA, MAT1, asd-3, preA and preB, and genes in uncharacterized SM gene clusters 30 and 44 showed increased expression in Hi-Fert-Mated sclerotia at these time points. These results broaden our knowledge of the biochemical and transcriptional processes during sexual development in A. flavus.}, number={3}, journal={FUNGAL BIOLOGY}, author={Luis, Jane Marian and Carbone, Ignazio and Mack, Brian M. and Lebar, Matthew D. and Cary, Jeffrey W. and Gilbert, Matthew K. and Bhatnagar, Deepak and Wientjes, Carol-Carter and Payne, Gary A. and Moore, Geromy G. and et al.}, year={2022}, month={Mar}, pages={187–200} }
 @article{kikway_keinath_ojiambo_2022, title={Field Occurrence and Overwintering of Oospores of Pseudoperonospora cubensis in the Southeastern United States}, volume={8}, ISSN={["1943-7684"]}, DOI={10.1094/PHYTO-11-21-0467-R}, abstractNote={ In the United States, the cucurbit downy mildew pathogen, Pseudoperonospora cubensis, has been shown to form oospores under laboratory conditions, but there are no reports on the formation of oospores in naturally infected cucurbit plants in the field. This study investigated the occurrence of oospores in naturally infected leaves from cucurbit fields in North Carolina and South Carolina from 2018 to 2020. Oospore viability and survival was also determined outdoors during the winter in North Carolina during this study period using soil containing leaves infested with oospores. About 5% of 1,658 naturally infected cucumber and cantaloupe leaves sampled during the study had oospores, with a mean density of 585 oospores per cm2 of infected leaf tissue. Absolute oospore viability, as assessed using the plasmolysis method, declined linearly (slope = −0.27; P < 0.0001) over the 6-month exposure period from 67.8% in November to 19.3% in May. Other variables being equal, the decrease in oospore viability was significantly affected by soil temperature (b = −0.03 to −0.05; P < 0.0001) and number of rainy days (b = 21.6 to 40.46; P < 0.05), while the effects of soil moisture on oospore viability were less clear. About 20% of the oospores exposed to outdoor conditions at the end the study period were putatively viable and deemed potentially infective. However, these putatively viable oospores failed to germinate or initiate disease when inoculated onto cucumber or cantaloupe leaves. These results indicate that oospores might require some unrecognized stimuli or physiological factors to initiate germination and infection. Nonetheless, viability of oospores at the end of the winter season suggests that once exposed to the right conditions that stimulate germination, these oospores could potentially serve as a primary inoculum source in the southeastern United States where winter temperatures are cold enough to kill cucurbits plants. }, journal={PHYTOPATHOLOGY}, author={Kikway, Isaack and Keinath, Anthony P. and Ojiambo, Peter S.}, year={2022}, month={Aug} }
 @article{nyanapah_ayiecho_nyabundi_otieno_ojiambo_2022, title={Genetic association of agronomic traits with partial resistance to gray leaf spot in elite maize germplasm}, volume={10}, ISSN={["1435-0653"]}, DOI={10.1002/csc2.20792}, abstractNote={Abstract}, journal={CROP SCIENCE}, author={Nyanapah, James O. and Ayiecho, Patrick O. and Nyabundi, Julius O. and Otieno, Washington and Ojiambo, Peter S.}, year={2022}, month={Oct} }
 @article{atehnkeng_ojiambo_ortega-beltran_augusto_cotty_bandyopadhyay_2022, title={Impact of frequency of application on the long-term efficacy of the biocontrol product Aflasafe in reducing aflatoxin contamination in maize}, volume={13}, ISSN={["1664-302X"]}, DOI={10.3389/fmicb.2022.1049013}, abstractNote={Aflatoxins, produced by severalAspergillussectionFlavispecies in various crops, are a significant public health risk and a barrier to trade and development. In sub-Saharan Africa, maize and groundnut are particularly vulnerable to aflatoxin contamination. Aflasafe, a registered aflatoxin biocontrol product, utilizes atoxigenicA. flavusgenotypes native to Nigeria to displace aflatoxin producers and mitigate aflatoxin contamination. Aflasafe was evaluated in farmers’ fields for 3 years, under various regimens, to quantify carry-over of the biocontrol active ingredient genotypes. Nine maize fields were each treated either continuously for 3 years, the first two successive years, in year 1 and year 3, or once during the first year. For each treated field, a nearby untreated field was monitored. Aflatoxins were quantified in grain at harvest and after simulated poor storage. Biocontrol efficacy and frequencies of the active ingredient genotypes decreased in the absence of annual treatment. Maize treated consecutively for 2 or 3 years had significantly (p < 0.05) less aflatoxin (92% less) in grain at harvest than untreated maize. Maize grain from treated fields subjected to simulated poor storage had significantly less (p < 0.05) aflatoxin than grain from untreated fields, regardless of application regimen. Active ingredients occurred at higher frequencies in soil and grain from treated fields than from untreated fields. The incidence of active ingredients recovered in soil was significantly correlated (r = 0.898;p < 0.001) with the incidence of active ingredients in grain, which in turn was also significantly correlated (r = −0.621,p = 0.02) with aflatoxin concentration. Although there were carry-over effects, caution should be taken when drawing recommendations about discontinuing biocontrol use. Cost–benefit analyses of single season and carry-over influences are needed to optimize use by communities of smallholder farmers in sub-Saharan Africa.}, journal={FRONTIERS IN MICROBIOLOGY}, author={Atehnkeng, Joseph and Ojiambo, Peter S. and Ortega-Beltran, Alejandro and Augusto, Joao and Cotty, Peter J. and Bandyopadhyay, Ranajit}, year={2022}, month={Nov} }
 @article{kikway_keinath_ojiambo_2022, title={Temporal Dynamics and Severity of Cucurbit Downy Mildew Epidemics as Affected by Chemical Control and Cucurbit Host Type}, volume={106}, ISSN={["1943-7692"]}, DOI={10.1094/PDIS-09-21-1992-RE}, abstractNote={ Cucurbit downy mildew caused by the oomycete Pseudoperonospora cubensis is an important disease that affects members of Cucurbitaceae family globally. However, temporal dynamics of the disease have not been characterized at the field scale to understand how control strategies influence disease epidemics. Disease severity was assessed visually on cucumber and summer squash treated with weekly alternation of chlorothalonil with cymoxanil, fluopicolide, or propamocarb during the 2018 spring season and 2019 and 2020 fall seasons in North Carolina and the 2018 and 2020 fall seasons in South Carolina. Disease onset was observed around mid-June during the spring season and early September during the fall season, followed by a rapid increase in severity until mid-July in the spring season and late September or mid-October in the fall season, typical of polycyclic epidemics. The Gompertz, logistic, and monomolecular growth models were fitted to disease severity using linear regression and parameter estimates to compare the effects of fungicide treatment and cucurbit host type on disease progress. The Gompertz and logistic models were more appropriate than the monomolecular model in describing temporal dynamics of cucurbit downy mildew, with the Gompertz model providing the best description for 34 of the 44 epidemics examined. Fungicide treatment and host type significantly (P < 0.0001) affected the standardized area under disease progress curve (sAUDPC), final disease severity (Final DS), and weighted mean absolute rates of disease progress (ρ), with these variables, in most cases, being significantly (P < 0.05) lower in fungicide-treated plots than in untreated control plots. Except in a few cases, sAUDPC, Final DS, and ρ were lower in cases where chlorothalonil was alternated with fluopicolide or propamocarb than in cases where chlorothalonil was alternated with cymoxanil or when chlorothalonil was applied alone. These results characterized the temporal progress of cucurbit downy mildew and provided an improved understanding of the dynamics of the disease at the field level. Parameters of disease progress obtained from this study could serve as inputs in simulation studies to assess the efficacy of fungicide alternation in managing fungicide resistance in this pathosystem. }, number={3}, journal={PLANT DISEASE}, author={Kikway, Isaack and Keinath, Anthony P. and Ojiambo, Peter S.}, year={2022}, month={Mar}, pages={1009–1019} }
 @article{nowicki_hadziabdic_trigiano_boggess_kanetis_wadl_ojiambo_cubeta_spring_thines_et al._2021, title={"Jumping Jack ": Genomic Microsatellites Underscore the Distinctiveness of Closely Related Pseudoperonospora cubensis and Pseudoperonospora humuli and Provide New Insights Into Their Evolutionary Past}, volume={12}, ISSN={["1664-302X"]}, DOI={10.3389/fmicb.2021.686759}, abstractNote={Downy mildews caused by obligate biotrophic oomycetes result in severe crop losses worldwide. Among these pathogens, Pseudoperonospora cubensis and P. humuli, two closely related oomycetes, adversely affect cucurbits and hop, respectively. Discordant hypotheses concerning their taxonomic relationships have been proposed based on host–pathogen interactions and specificity evidence and gene sequences of a few individuals, but population genetics evidence supporting these scenarios is missing. Furthermore, nuclear and mitochondrial regions of both pathogens have been analyzed using microsatellites and phylogenetically informative molecular markers, but extensive comparative population genetics research has not been done. Here, we genotyped 138 current and historical herbarium specimens of those two taxa using microsatellites (SSRs). Our goals were to assess genetic diversity and spatial distribution, to infer the evolutionary history of P. cubensis and P. humuli, and to visualize genome-scale organizational relationship between both pathogens. High genetic diversity, modest gene flow, and presence of population structure, particularly in P. cubensis, were observed. When tested for cross-amplification, 20 out of 27 P. cubensis-derived gSSRs cross-amplified DNA of P. humuli individuals, but few amplified DNA of downy mildew pathogens from related genera. Collectively, our analyses provided a definite argument for the hypothesis that both pathogens are distinct species, and suggested further speciation in the P. cubensis complex.}, journal={FRONTIERS IN MICROBIOLOGY}, author={Nowicki, Marcin and Hadziabdic, Denita and Trigiano, Robert N. and Boggess, Sarah L. and Kanetis, Loukas and Wadl, Phillip A. and Ojiambo, Peter S. and Cubeta, Marc A. and Spring, Otmar and Thines, Marco and et al.}, year={2021}, month={Jul} }
 @article{ojwang'_ruiz_bhattacharyya_chatterjee_ojiambo_gent_2021, title={A General Framework for Spatio-Temporal Modeling of Epidemics With Multiple Epicenters: Application to an Aerially Dispersed Plant Pathogen}, volume={7}, ISSN={["2297-4687"]}, DOI={10.3389/fams.2021.721352}, abstractNote={The spread dynamics of long-distance-dispersed pathogens are influenced by the dispersal characteristics of a pathogen, anisotropy due to multiple factors, and the presence of multiple sources of inoculum. In this research, we developed a flexible class of phenomenological spatio-temporal models that extend a modeling framework used in plant pathology applications to account for the presence of multiple sources and anisotropy of biological species that can govern disease gradients and spatial spread in time. We use the cucurbit downy mildew pathosystem (caused by Pseudoperonospora cubensis) to formulate a data-driven procedure based on the 2008 to 2010 historical occurrence of the disease in the U.S. available from standardized sentinel plots deployed as part of the Cucurbit Downy Mildew ipmPIPE program. This pathosystem is characterized by annual recolonization and extinction cycles, generating annual disease invasions at the continental scale. This data-driven procedure is amenable to fitting models of disease spread from one or multiple sources of primary inoculum and can be specified to provide estimates of the parameters by regression methods conditional on a function that can accommodate anisotropy in disease occurrence data. Applying this modeling framework to the cucurbit downy mildew data sets, we found a small but consistent reduction in temporal prediction errors by incorporating anisotropy in disease spread. Further, we did not find evidence of an annually occurring, alternative source of P. cubensis in northern latitudes. However, we found a signal indicating an alternative inoculum source on the western edge of the Gulf of Mexico. This modeling framework is tractable for estimating the generalized location and velocity of a disease front from sparsely sampled data with minimal data acquisition costs. These attributes make this framework applicable and useful for a broad range of ecological data sets where multiple sources of disease may exist and whose subsequent spread is directional.}, journal={FRONTIERS IN APPLIED MATHEMATICS AND STATISTICS}, author={Ojwang', Awino M. E. and Ruiz, Trevor and Bhattacharyya, Sharmodeep and Chatterjee, Shirshendu and Ojiambo, Peter S. and Gent, David H.}, year={2021}, month={Nov} }
 @misc{copes_ojiambo_2021, title={Efficacy of Hypochlorite as a Disinfestant Against Fungal Pathogens in Agricultural and Horticultural Plant Production: A Systematic Review and Meta-Analysis}, volume={111}, ISSN={["1943-7684"]}, DOI={10.1094/PHYTO-05-20-0201-R}, abstractNote={ Hypochlorite is often used as a disinfestant of fungal pathogens in a range of agricultural and horticultural settings. However, reports of its effectiveness are variable across studies and it is unclear what factors could potentially influence the reported estimates of its efficacy. A systematic review and meta-analysis was conducted to assess the efficacy of hypochlorite against fungal pathogens and explore factors that may explain the observed heterogeneity in estimates of efficacy. Standardized mean effect size, Hedges’ g, was calculated for each of the 109 selected studies, published from 1972 to 2019, that met the criteria defined for the systematic review. A random-effects model was used to estimate the overall mean effect size ([Formula: see text]) and determine the heterogeneity in g among studies. Hypochlorite resulted in a significant (P < 0.001) reduction in either disease intensity or propagule viability with [Formula: see text]= 2.25, suggesting a large overall effect. However, 95% prediction intervals ranged from −0.18 to 4.68, indicating that hypochlorite could be ineffective against some fungi or when targeting some substrate materials. An estimate of the within-study variability, τ2, was 1.48 and the proportion of heterogeneity in g among studies due to true effects was 71.5%. Inclusion of categorical moderator variables in the random effects model showed that hypochlorite treatments were significantly (P < 0.0062) more effective when used to disinfest spores in an aqueous solution ([Formula: see text]= 4.58) than when used on plastic ([Formula: see text]= 2.13), plant ([Formula: see text]= 2.13), and wood ([Formula: see text]= 0.79). Similarly, hypochlorite treatments were significantly (P < 0.0083) more effective in disinfesting fungal propagules of Thielaviopsis spp. ([Formula: see text]= 2.51) than those of Verticillium spp. ([Formula: see text]= 1.21). A meta-regression indicated that the effect of dose (β = −3.54; P = 0.0398) and contact time (β = −0.05; P = 0.0001) on [Formula: see text] were highly significant. Further, [Formula: see text]was significantly affected by the dose × time interaction (β = −0.017; P = 0.0269). In the meta-regression models, dose and time explained 0 and 16% of the variance in true effects, respectively. In meta-regression models with a continuous variable of dose or time, a categorical variable of target or genus and their interaction term, genus and target explained an additional 7 to 19% of the variance in true effects. These results show that although the current recommended dose and contact time for commercial bleach products are expected to result in effective disinfestation, the target material and genera of the fungal pathogen of interest will likely influence their efficacy. }, number={8}, journal={PHYTOPATHOLOGY}, author={Copes, Warren E. and Ojiambo, Peter S.}, year={2021}, month={Aug}, pages={1369–1379} }
 @article{copes_ojiambo_2021, title={Efficacy of Hypochlorite in Disinfesting Nonfungal Plant Pathogens in Agricultural and Horticultural Plant Production: A Meta-Analysis}, volume={105}, ISSN={["1943-7692"]}, DOI={10.1094/PDIS-09-20-2046-RE}, abstractNote={ Bleach products containing hypochlorite are commonly used as disinfectants to eliminate nonfungal plant pathogens from production surfaces, tools, plant surfaces, irrigation water, and produce dump tanks. Although bleach products are useful, their effectiveness has been reported to vary under specific settings. A meta-analysis was conducted of 86 studies to assess the overall efficacy of hypochlorite against plant pathogenic bacteria, oomycetes, and viruses and to identify factors that explain differences in product efficacy. Hypochlorite resulted in a significant (P < 0.0001) reduction in disease intensity or propagule viability, with a mean Hedges’ g standardized difference ([Formula: see text]) of 3.01, indicating that overall, hypochlorite treatments are highly effective. However, heterogeneity in g was significant (P < 0.0001) between studies, wherein 69.8% of the variance observed in g was attributed to true effects. Furthermore, an estimate of between-study variability was moderate (τ2 = 1.46). Random effects (REs) metaregression showed limited effects of moderator variables dosage, contact time, targeted material of treatment, and organism type on product efficacy when all organism types were considered together. Because subgroup [Formula: see text] was significantly higher (P = 0.0070) for oomycetes ([Formula: see text] = 3.30) than for bacteria ([Formula: see text] = 2.19), subsequent metaregressions were performed by organism type. For oomycetes, five RE metaregression models, each containing two moderators and their interaction, resulted in significant (P = 0.05) effects, where models with dosage and time, dosage and genus, time and genus, dosage and target, and time and target accounted for ≤50, 71, 57, 48, and 47%, respectively, of the variance in true effect sizes (R2) associated with [Formula: see text]. For viruses, only the RE metaregression model containing time and target and their interaction resulted in significant (P = 0.0435) effects accounting for 38% of the variance in true effect sizes associated with [Formula: see text]. None of the RE metaregression models for bacteria were significant, although they still accounted for ≤28% of the variance in true effect sizes associated with [Formula: see text]. These results show that although the current recommended rates for dosage and contact time for commercial bleach products are generally expected to result in effective disinfestation, the efficacy against nonfungal plant pathogens is expected to be influenced by the organism type and target being treated with hypochlorite. }, number={12}, journal={PLANT DISEASE}, author={Copes, Warren E. and Ojiambo, Peter S.}, year={2021}, month={Dec}, pages={4084–4094} }
 @article{sharma_timilsina_abrahamian_minsavage_colee_ojiambo_goss_vallad_jones_2021, title={Need for speed: bacterial effector XopJ2 is associated with increased dispersal velocity of Xanthomonas perforans}, volume={6}, ISSN={["1462-2920"]}, DOI={10.1111/1462-2920.15541}, abstractNote={Summary}, journal={ENVIRONMENTAL MICROBIOLOGY}, author={Sharma, Anuj and Timilsina, Sujan and Abrahamian, Peter and Minsavage, Gerald V and Colee, James and Ojiambo, Peter S. and Goss, Erica M. and Vallad, Gary E. and Jones, Jeffrey B.}, year={2021}, month={Jun} }
 @article{luis_carbone_payne_bhatnagar_cary_moore_lebar_wei_mack_ojiambo_2020, title={Characterization of morphological changes within stromata during sexual reproduction inAspergillus flavus}, volume={112}, ISSN={["1557-2536"]}, DOI={10.1080/00275514.2020.1800361}, abstractNote={ABSTRACT Aspergillus flavus contaminates agricultural products worldwide with carcinogenic aflatoxins that pose a serious health risk to humans and animals. The fungus survives adverse environmental conditions through production of sclerotia. When fertilized by a compatible conidium of an opposite mating type, a sclerotium transforms into a stroma within which ascocarps, asci, and ascospores are formed. However, the transition from a sclerotium to a stroma during sexual reproduction in A. flavus is not well understood. Early events during the interaction between sexually compatible strains of A. flavus were visualized using conidia of a green fluorescent protein (GFP)-labeled MAT1-1 strain and sclerotia of an mCherry-labeled MAT1-2 strain. Both conidia and sclerotia of transformed strains germinated to produce hyphae within 24 h of incubation. Hyphal growth of these two strains produced what appeared to be a network of interlocking hyphal strands that were observed at the base of the mCherry-labeled sclerotia (i.e., region in contact with agar surface) after 72 h of incubation. At 5 wk following incubation, intracellular green-fluorescent hyphal strands were observed within the stromatal matrix of the mCherry-labeled strain. Scanning electron microscopy of stromata from a high- and low-fertility cross and unmated sclerotia was used to visualize the formation and development of sexual structures within the stromatal and sclerotial matrices, starting at the time of crossing and thereafter every 2 wk until 8 wk of incubation. Morphological differences between sclerotia and stromata became apparent at 4 wk of incubation. Internal hyphae and croziers were detected inside multiple ascocarps that developed within the stromatal matrix of the high-fertility cross but were not detected in the matrix of the low-fertility cross or the unmated sclerotia. At 6 to 8 wk of incubation, hyphal tips produced numerous asci, each containing one to eight ascospores that emerged out of an ascus following the breakdown of the ascus wall. These observations broaden our knowledge of early events during sexual reproduction and suggest that hyphae from the conidium-producing strain may be involved in the early stages of sexual reproduction in A. flavus. When combined with omics data, these findings could be useful in further exploration of the molecular and biochemical mechanisms underlying sexual reproduction in A. flavus.}, number={5}, journal={MYCOLOGIA}, author={Luis, Jane Marian and Carbone, Ignazio and Payne, Gary A. and Bhatnagar, Deepak and Cary, Jeffrey W. and Moore, Geromy G. and Lebar, Matthew D. and Wei, Qijian and Mack, Brian and Ojiambo, Peter S.}, year={2020}, month={Sep}, pages={908–920} }
 @article{nyanapah_ayiecho_nyabundi_otieno_ojiambo_2020, title={Field Characterization of Partial Resistance to Gray Leaf Spot in Elite Maize Germplasm}, volume={110}, ISSN={["1943-7684"]}, DOI={10.1094/PHYTO-12-19-0446-R}, abstractNote={Forty-eight inbred lines of maize with varying levels of resistance to gray leaf spot (GLS) were artificially inoculated with Cercospora zeina and evaluated to characterize partial disease resistance in maize under field conditions from 2012 to 2014 across 12 environments in western Kenya. Eight measures of disease epidemic—that is, final percent diseased leaf area (FPDLA), standardized area under the disease progress curve (SAUDPC), weighted mean absolute rate of disease increase (ρ), disease severity scale (CDSG), percent diseased leaf area at the inflection point (PDLAIP), SAUDPC at the inflection point (SAUDPCIP), time from inoculation to transition of disease progress from the increasing to the decreasing phase of epidemic increase (TIP), and latent period (LP)—were examined. Inbred lines significantly (P < 0.05) affected all measures of disease epidemic except ρ. However, the proportion of the variation attributed to the analysis of variance model was most strongly associated with SAUDPC (R2= 89.4%). Inbred lines were also most consistently ranked for disease resistance based on SAUDPC. Although SAUDPC was deemed the most useful variable for quantifying partial resistance in the test genotypes, the proportion of the variation in SAUDPC in each plot was most strongly (R2= 93.9%) explained by disease ratings taken between the VT and R4 stages of plant development. Individual disease ratings at the R4 stage of plant development were nearly as effective as SAUDPC in discerning the differential reaction of test genotypes. Thus, GLS rankings of inbred lines based on disease ratings at these plant developmental stages should be useful in prebreeding nurseries and preliminary evaluation trials involving large germplasm populations.}, number={10}, journal={PHYTOPATHOLOGY}, author={Nyanapah, James O. and Ayiecho, Patrick O. and Nyabundi, Julius O. and Otieno, Washington and Ojiambo, Peter S.}, year={2020}, month={Oct}, pages={1668–1679} }
 @article{lewis_carbone_luis_payne_bowen_hagan_kemerait_heiniger_ojiambo_2019, title={Biocontrol Strains Differentially Shift the Genetic Structure of Indigenous Soil Populations of Aspergillus flavus}, volume={10}, ISSN={1664-302X}, url={http://dx.doi.org/10.3389/fmicb.2019.01738}, DOI={10.3389/fmicb.2019.01738}, abstractNote={Biocontrol using non-aflatoxigenic strains of Aspergillus flavus has the greatest potential to mitigate aflatoxin contamination in agricultural produce. However, factors that influence the efficacy of biocontrol agents in reducing aflatoxin accumulation under field conditions are not well-understood. Shifts in the genetic structure of indigenous soil populations of A. flavus following application of biocontrol products Afla-Guard and AF36 were investigated to determine how these changes can influence the efficacy of biocontrol strains in reducing aflatoxin contamination. Soil samples were collected from maize fields in Alabama, Georgia, and North Carolina in 2012 and 2013 to determine changes in the population genetic structure of A. flavus in the soil following application of the biocontrol strains. A. flavus L was the most dominant species of Aspergillus section Flavi with a frequency ranging from 61 to 100%, followed by Aspergillus parasiticus that had a frequency of <35%. The frequency of A. flavus L increased, while that of A. parasiticus decreased after application of biocontrol strains. A total of 112 multilocus haplotypes (MLHs) were inferred from 1,282 isolates of A. flavus L using multilocus sequence typing of the trpC, mfs, and AF17 loci. A. flavus individuals belonging to the Afla-Guard MLH in the IB lineage were the most dominant before and after application of biocontrol strains, while individuals of the AF36 MLH in the IC lineage were either recovered in very low frequencies or not recovered at harvest. There were no significant (P > 0.05) differences in the frequency of individuals with MAT1-1 and MAT1-2 for clone-corrected MLH data, an indication of a recombining population resulting from sexual reproduction. Population mean mutation rates were not different across temporal and spatial scales indicating that mutation alone is not a driving force in observed multilocus sequence diversity. Clustering based on principal component analysis identified two distinct evolutionary lineages (IB and IC) across all three states. Additionally, patristic distance analysis revealed phylogenetic incongruency among single locus phylogenies which suggests ongoing genetic exchange and recombination. Levels of aflatoxin accumulation were very low except in North Carolina in 2012, where aflatoxin levels were significantly (P < 0.05) lower in grain from treated compared to untreated plots. Phylogenetic analysis showed that Afla-Guard was more effective than AF36 in shifting the indigenous soil populations of A. flavus toward the non-toxigenic or low aflatoxin producing IB lineage. These results suggest that Afla-Guard, which matches the genetic and ecological structure of indigenous soil populations of A. flavus in Alabama, Georgia, and North Carolina, is likely to be more effective in reducing aflatoxin accumulation and will also persist longer in the soil than AF36 in the southeastern United States.}, journal={Frontiers in Microbiology}, publisher={Frontiers Media SA}, author={Lewis, Mary H. and Carbone, Ignazio and Luis, Jane M. and Payne, Gary A. and Bowen, Kira L. and Hagan, Austin K. and Kemerait, Robert and Heiniger, Ron and Ojiambo, Peter S.}, year={2019}, month={Jul} }
 @article{kinyungu_isakeit_ojiambo_woloshuk_2019, title={Spread of Aspergillus flavus and aflatoxin accumulation in postharvested maize treated with biocontrol products}, volume={84}, DOI={10.1016/j.jspr.2019.101519}, abstractNote={Maize is a major staple crop and calorie source for many people living in Sub-Saharan Africa. In this region, Aspergillus flavus causes ear rot in maize, contributing to food insecurity due to aflatoxin contamination. The biological control principle of competitive exclusion has been applied in both the United States and Africa to reduce aflatoxin levels in maize grain at harvest by introducing atoxigenic strains that out-compete toxigenic strains. The goal of this study was to determine if the efficacy of preharvest biocontrol treatments carry over into the postharvest drying period, the time between harvest and the point when grain moisture is safe for storage. In Sub-Sahara Africa, this period often is extended by weather and the complexities of postharvest drying practices. Maize grain was collected from fields in Texas and North Carolina that were treated with commercial biocontrol products and untreated control fields. To simulate moisture conditions similar to those experienced by farmers during drying in Sub-Sahara Africa, we adjusted the grain to 20% moisture content and incubated it at 28 °C for 6 days. Although the initial number of kernels infected by fungal species was high in most samples, less than 24% of kernels were infected with Aspergillus flavus and aflatoxin levels were low (<4 ppb). Both toxigenic and atoxigenic strains grew and spread through the grain over the incubation period, and aflatoxin levels increased, even in samples from biocontrol-treated fields. Our molecular analysis suggests that applied biocontrol strains from treated fields may have migrated to untreated fields. These results also indicate that the population of toxigenic A. flavus in the harvested grain will increase and produce aflatoxin during the drying period when moisture is high. Therefore, we conclude that preharvest biocontrol applications will not replace the need for better postharvest practices that reduce the drying time between harvest and storage.}, journal={Journal of Stored Products Research}, publisher={Elsevier BV}, author={Kinyungu, Sharon and Isakeit, Tom and Ojiambo, Peter S. and Woloshuk, Charles P.}, year={2019}, pages={101519} }
 @article{ojiambo_battilani_cary_blum_carbone_2018, title={Cultural and Genetic Approaches to Manage Aflatoxin Contamination: Recent Insights Provide Opportunities for Improved Control}, volume={108}, ISSN={0031-949X}, url={http://dx.doi.org/10.1094/PHYTO-04-18-0134-RVW}, DOI={10.1094/PHYTO-04-18-0134-RVW}, abstractNote={ Aspergillus flavus is a morphologically complex species that can produce the group of polyketide derived carcinogenic and mutagenic secondary metabolites, aflatoxins, as well as other secondary metabolites such as cyclopiazonic acid and aflatrem. Aflatoxin causes aflatoxicosis when aflatoxins are ingested through contaminated food and feed. In addition, aflatoxin contamination is a major problem, from both an economic and health aspect, in developing countries, especially Asia and Africa, where cereals and peanuts are important food crops. Earlier measures for control of A. flavus infection and consequent aflatoxin contamination centered on creating unfavorable environments for the pathogen and destroying contaminated products. While development of atoxigenic (nonaflatoxin producing) strains of A. flavus as viable commercial biocontrol agents has marked a unique advance for control of aflatoxin contamination, particularly in Africa, new insights into the biology and sexuality of A. flavus are now providing opportunities to design improved atoxigenic strains for sustainable biological control of aflatoxin. Further, progress in the use of molecular technologies such as incorporation of antifungal genes in the host and host-induced gene silencing, is providing knowledge that could be harnessed to develop germplasm that is resistant to infection by A. flavus and aflatoxin contamination. This review summarizes the substantial progress that has been made to understand the biology of A. flavus and mitigate aflatoxin contamination with emphasis on maize. Concepts developed to date can provide a basis for future research efforts on the sustainable management of aflatoxin contamination. }, number={9}, journal={Phytopathology}, publisher={Scientific Societies}, author={Ojiambo, Peter S. and Battilani, Paola and Cary, Jeffrey W. and Blum, Burt H. and Carbone, Ignazio}, year={2018}, month={Sep}, pages={1024–1037} }
 @article{neufeld_keinath_ojiambo_2018, title={Evaluation of a Model for Predicting the Infection Risk of Squash and Cantaloupe by Pseudoperonospora cubensis}, volume={102}, ISSN={["1943-7692"]}, DOI={10.1094/pdis-07-17-1046-re}, abstractNote={ Infection risk models of downy mildew of cucumber caused by Pseudoperonospora cubensis were evaluated for their performance in predicting the infection risk of squash and cantaloupe plants under field conditions. Experiments were conducted from 2012 to 2014 in Clayton, NC and Charleston, SC, where disease-free potted plants were exposed to weather conditions during a 24- and 48-h period (hereafter 24- and 48-h models, respectively) within a plot with naturally occurring inoculum. Exposed plants were subsequently placed in a growth chamber where they were monitored for disease symptoms, which was indicative of a successful infection. Disease severity was assessed after 7 days as the proportion of leaf area with disease symptoms. Two predictor variables, day temperature and hours of relative humidity >80% during each exposure were used as inputs to generate model predictions that were compared with observed data. The threshold probability on the receiver operating characteristic (ROC) curve that minimized the overall error rate for the 24-h model was 0.85 for both squash and cantaloupe. The 24-h model was consistently more accurate than the 48-h model in predicting the infection risk for the two hosts. The accuracy of the 24-h model as estimated using area under ROC curve ranged from 0.75 to 0.81, with a correct classification rate ranging from 0.69 to 0.74 across the two hosts. Specificity rates for the model ranged from 0.81 to 0.84, while the sensitivity rates ranged from 0.58 to 0.67. Optimal decisions thresholds (POT) developed based on estimates of economic damage and costs of management showed that POT was dependent on the probability of disease occurrence, with the benefit of using the 24-h model for making management decisions being greatest at low levels of probability of disease occurrence. This 24-h model, previously developed using cucumber as the host, resulted in accurate estimates of the daily infection risk of squash and cantaloupe and could potentially be useful when incorporated into a decision support tool to guide fungicide applications to manage downy mildew in these other cucurbit host types. }, number={5}, journal={PLANT DISEASE}, publisher={Scientific Societies}, author={Neufeld, Katie N. and Keinath, Anthony P. and Ojiambo, Peter S.}, year={2018}, month={May}, pages={855–862} }
 @article{neufeld_keinath_gugino_mcgrath_sikora_miller_ivey_langston_dutta_keever_et al._2018, title={Predicting the risk of cucurbit downy mildew in the eastern United States using an integrated aerobiological model}, volume={62}, ISSN={["1432-1254"]}, url={http://europepmc.org/abstract/med/29177798}, DOI={10.1007/s00484-017-1474-2}, abstractNote={Cucurbit downy mildew caused by the obligate oomycete, Pseudoperonospora cubensis, is considered one of the most economically important diseases of cucurbits worldwide. In the continental United States, the pathogen overwinters in southern Florida and along the coast of the Gulf of Mexico. Outbreaks of the disease in northern states occur annually via long-distance aerial transport of sporangia from infected source fields. An integrated aerobiological modeling system has been developed to predict the risk of disease occurrence and to facilitate timely use of fungicides for disease management. The forecasting system, which combines information on known inoculum sources, long-distance atmospheric spore transport and spore deposition modules, was tested to determine its accuracy in predicting risk of disease outbreak. Rainwater samples at disease monitoring sites in Alabama, Georgia, Louisiana, New York, North Carolina, Ohio, Pennsylvania and South Carolina were collected weekly from planting to the first appearance of symptoms at the field sites during the 2013, 2014, and 2015 growing seasons. A conventional PCR assay with primers specific to P. cubensis was used to detect the presence of sporangia in rain water samples. Disease forecasts were monitored and recorded for each site after each rain event until initial disease symptoms appeared. The pathogen was detected in 38 of the 187 rainwater samples collected during the study period. The forecasting system correctly predicted the risk of disease outbreak based on the presence of sporangia or appearance of initial disease symptoms with an overall accuracy rate of 66 and 75%, respectively. In addition, the probability that the forecasting system correctly classified the presence or absence of disease was ≥ 73%. The true skill statistic calculated based on the appearance of disease symptoms in cucurbit field plantings ranged from 0.42 to 0.58, indicating that the disease forecasting system had an acceptable to good performance in predicting the risk of cucurbit downy mildew outbreak in the eastern United States.}, number={4}, journal={INTERNATIONAL JOURNAL OF BIOMETEOROLOGY}, author={Neufeld, K. N. and Keinath, A. P. and Gugino, B. K. and McGrath, M. T. and Sikora, E. J. and Miller, S. A. and Ivey, M. L. and Langston, D. B. and Dutta, B. and Keever, T. and et al.}, year={2018}, month={Apr}, pages={655–668} }
 @article{rsaliyev_amirkhanova_rametov_pahratdinova_ojiambo_lebeda_2018, title={Pseudoperonospora cubensis virulence and pathotype structure in Kazakhstan}, volume={67}, ISSN={["1365-3059"]}, url={https://doi.org/10.1111/ppa.12890}, DOI={10.1111/ppa.12890}, abstractNote={Severe epidemics of cucurbit downy mildew caused by Pseudoperonospora cubensis have been observed in recent years in major cucumber‐producing regions in Kazakhstan. However, information on the pathogenic variability of P. cubensis that could facilitate screening of candidate cucumber varieties and hybrids prior to deployment for commercial production is currently not available. A total of 112 isolates were collected from 2014 to 2016 during surveys in three major cucumber‐producing regions of Kazakhstan. Single lesion isolates were tested for their compatibility with a differential host set composed of 12 genotypes. Most isolates were compatible with Cucumis sativus and C. melo subsp. melo, while none were compatible with Citrullus lanatus. Isolates differed primarily by their virulence to C. pepo subsp. pepo and Benincasa hispida. Based on the patterns of compatibility with the differential host set, 29 pathotypes were identified. Of these, 18 were associated with a specific production region, six occurred in two regions and five were present in all three regions. The number of virulence factors (VF) ranged from 2 to 10, with 65% of the pathotypes having eight or more VFs. Indices of diversity showed that virulence was very diverse within the pathogen populations. Clustering of pathotypes based on similarity in virulence showed the existence of two main clusters. The high pathogenic variability observed in this study suggests that newly developed cucumber varieties and hybrids with resistance to cucurbit downy mildew need to be evaluated against a wide range of pathotypes prior to their deployment for commercial production in Kazakhstan.}, number={9}, journal={PLANT PATHOLOGY}, publisher={Wiley}, author={Rsaliyev, A. S. and Amirkhanova, N. T. and Rametov, N. M. and Pahratdinova, Z. U. and Ojiambo, P. S. and Lebeda, A.}, year={2018}, month={Dec}, pages={1924–1935} }
 @article{thomas_neufeld_seebold_braun_schwarz_ojiambo_2018, title={Resistance to Fluopicolide and Propamocarb and Baseline Sensitivity to Ethaboxam Among Isolates of Pseudoperonospora cubensis From the Eastern United States}, volume={102}, url={https://doi.org/10.1094/PDIS-10-17-1673-RE}, DOI={10.1094/PDIS-10-17-1673-RE}, abstractNote={ Chemical control is currently the most effective method for controlling cucurbit downy mildew (CDM) caused by Pseudoperonospora cubensis. Most commercial cucurbit cultivars, with the exception of a few new cucumber cultivars, lack adequate disease resistance. Fluopicolide and propamocarb were among the most effective fungicides against CDM in the United States between 2006 and 2009. Since then, reduced efficacy of these two fungicides under field conditions was reported starting around 2013 but occurrence of resistance to fluopicolide and propamocarb in field isolates of P. cubensis had not been established. Thirty-one isolates collected from cucurbits in the eastern United States were tested for their sensitivity to fluopicolide and propamocarb using a leaf disc assay. This same set of isolates and four additional isolates (i.e., 35 isolates) were also used to establish the baseline sensitivity of P. cubensis to ethaboxam, an ethylamino-thiazole-carboxamide fungicide, which was recently granted registration to control CDM in the United States. About 65% of the isolates tested were resistant to fluopicolide with at least one resistant isolate being present in samples collected from 12 of the 13 states in the eastern United States. About 74% of the isolates tested were sensitive to propamocarb with at least one resistant isolate being among samples collected from 8 of the 12 states in the study. The frequency of resistance to fluopicolide and propamocarb was high among isolates collected from cucumber, while the frequency was low among isolates collected from other cucurbit host types. All isolates tested were found to be sensitive to ethaboxam and EC50 values ranged from 0.18 to 3.08 mg a.i./liter with a median of 1.55 mg a.i./liter. The ratio of EC50 values for the least sensitive and the most sensitive isolate was 17.1, indicating that P. cubensis isolates were highly sensitive to ethaboxam. The most sensitive isolates to ethaboxam were collected from New York, North Carolina, and Ohio, while the least sensitive isolates were collected from Georgia, Michigan, and New Jersey. These results show that ethaboxam could be a viable addition to fungicide programs used to control CDM in the United States. }, number={8}, journal={Plant Disease}, publisher={Scientific Societies}, author={Thomas, A. and Neufeld, K. N. and Seebold, K. W. and Braun, C. A. and Schwarz, M. R. and Ojiambo, P. S.}, year={2018}, month={Aug}, pages={1619–1626} }
 @article{mehra_adhikari_cowger_ojiambo_2018, title={Septoria nodorum blotch of wheat}, volume={7}, url={http://europepmc.org/abstract/PPR/PPR45283}, DOI={10.7287/peerj.preprints.27039v1}, abstractNote={Septoria nodorum blotch occurs in wheat-growing areas worldwide, but the disease is more prevalent in areas with warm and moist weather, such as the southeastern United States, parts of Europe, southern Brazil, and Australia. The disease affects both the quantity and quality of yield, and the pathogen is capable of affecting wheat at both seedling and adult stages. Historically, losses up to 50% have been reported, in addition to lower grain quality, although in the U.S., lower levels of loss are typical. The yield losses are highest when flag leaf, F-1 (leaf below flag leaf), and F-2 (leaf below F-1) are infected. The disease is known to reduce thousand-kernel-weight, a yield parameter. The fungus undergoes regular cycles of sexual recombination due to the availability of both mating types, and creates genetic variation in its population, thus enhancing its potential to overcome control measures. The pathosystem is also a model system for necrotrophic plant pathogens. So far, nine necrotrophic effectors and host susceptibility gene interaction have been identified, which have the potential to be used in marker assisted selection for breeding resistant wheat varieties.}, publisher={PeerJ}, author={Mehra, Lucky and Adhikari, Urmila and Cowger, Christina and Ojiambo, Peter S}, year={2018}, month={Jul} }
 @article{mehra_adhikari_cowger_ojiambo_2018, title={Septoria nodorum blotch of wheat}, volume={7}, url={http://europepmc.org/abstract/PPR/PPR96938}, DOI={10.7287/peerj.preprints.27039v2}, abstractNote={Septoria nodorum blotch occurs in wheat-growing areas worldwide, but the disease is more prevalent in areas with warm and moist weather, such as the southeastern United States, parts of Europe, southern Brazil, and Australia. The disease affects both the quantity and quality of yield, and the pathogen is capable of affecting wheat at both seedling and adult stages. Historically, losses up to 50% have been reported, in addition to lower grain quality, although in the U.S., lower levels of loss are typical. The yield losses are highest when flag leaf, F-1 (leaf below flag leaf), and F-2 (leaf below F-1) are infected. The disease is known to reduce thousand-kernel-weight, a yield parameter. The fungus undergoes regular cycles of sexual recombination due to the availability of both mating types, and creates genetic variation in its population, thus enhancing its potential to overcome control measures. The pathosystem is also a model system for necrotrophic plant pathogens. So far, nine necrotrophic effectors and host susceptibility gene interaction have been identified, which have the potential to be used in marker assisted selection for breeding resistant wheat varieties.}, publisher={PeerJ}, author={Mehra, Lucky and Adhikari, Urmila and Cowger, Christina and Ojiambo, Peter S}, year={2018}, month={Jul} }
 @article{mehra_adhikari_cowger_ojiambo_2018, title={Septoria nodorum blotch of wheat}, volume={7}, DOI={10.7287/peerj.preprints.27039}, abstractNote={Septoria nodorum blotch occurs in wheat-growing areas worldwide, but the disease is more prevalent in areas with warm and moist weather, such as the southeastern United States, parts of Europe, southern Brazil, and Australia. The disease affects both the quantity and quality of yield, and the pathogen is capable of affecting wheat at both seedling and adult stages. Historically, losses up to 50% have been reported, in addition to lower grain quality, although in the U.S., lower levels of loss are typical. The yield losses are highest when flag leaf, F-1 (leaf below flag leaf), and F-2 (leaf below F-1) are infected. The disease is known to reduce thousand-kernel-weight, a yield parameter. The fungus undergoes regular cycles of sexual recombination due to the availability of both mating types, and creates genetic variation in its population, thus enhancing its potential to overcome control measures. The pathosystem is also a model system for necrotrophic plant pathogens. So far, nine necrotrophic effectors and host susceptibility gene interaction have been identified, which have the potential to be used in marker assisted selection for breeding resistant wheat varieties.}, publisher={PeerJ}, author={Mehra, Lucky and Adhikari, Urmila and Cowger, Christina and Ojiambo, Peter S}, year={2018}, month={Jul} }
 @article{mehra_cowger_ojiambo_2017, title={A Model for Predicting Onset of Stagonospora nodorum Blotch in Winter Wheat Based on Preplanting and Weather Factors}, volume={107}, ISSN={["1943-7684"]}, DOI={10.1094/phyto-03-16-0133-r}, abstractNote={ Stagonospora nodorum blotch (SNB) caused by Parastagonospora nodorum is a serious disease of wheat worldwide. In the United States, the disease is prevalent on winter wheat in many eastern states, and its management relies mainly on fungicide application after flag leaf emergence. Although SNB can occur prior to flag leaf emergence, the relationship between the time of disease onset and yield has not been determined. Such a relationship is useful in identifying a threshold to facilitate prediction of disease onset in the field. Disease occurred in 390 of 435 disease cases that were recorded across 11 counties in North Carolina from 2012 to 2014. Using cases with disease occurrence, the effect of disease onset on yield was analyzed to identify a disease onset threshold that related time of disease onset to yield. Regression analysis showed that disease onset explained 32% of the variation in yield (P < 0.0001) and from this relationship, day of year (DOY) 102 was identified as the disease onset threshold. Below-average yield occurred in 87% of the disease cases when disease onset occurred before DOY 102 but in only 28% of those cases when onset occurred on or after DOY 102. Subsequently, binary logistic regression models were developed to predict the occurrence and onset of SNB using preplanting factors and cumulative daily infection values (cDIV) starting 1 to 3 weeks prior to DOY 102. Logistic regression showed that previous crop, latitude, and cDIV accumulated 2 weeks prior to DOY 102 (cDIV.2) were significant (P < 0.0001) predictors of disease occurrence, and wheat residue, latitude, longitude, and cDIV.2 were significant (P < 0.0001) predictors of disease onset. The disease onset model had a correct classification rate of 0.94 and specificity and sensitivity rates >0.90. Performance of the disease onset model based on the area under the receiver operating characteristic curve (AUC), κ, and the true skill statistic (TSS) was excellent, with prediction accuracy values >0.88. Similarly, internal validation of the disease onset model based on AUC, κ, and TSS indicated good performance, with accuracy values >0.88. This disease onset prediction model could serve as a useful decision support tool to guide fungicide applications to manage SNB in wheat. }, number={6}, journal={PHYTOPATHOLOGY}, publisher={Scientific Societies}, author={Mehra, L. K. and Cowger, C. and Ojiambo, P. S.}, year={2017}, month={Jun}, pages={635–644} }
 @article{neufeld_keinath_ojiambo_2017, title={A model to predict the risk of infection of cucumber by Pseudoperonospora cubensis}, volume={6}, ISSN={2352-3522}, url={http://dx.doi.org/10.1016/J.MRAN.2017.05.001}, DOI={10.1016/J.MRAN.2017.05.001}, abstractNote={Infection of cucurbits by Pseudoperonospora cubensis and subsequent development of cucurbit downy mildew (CDM) is primarily driven by prevailing weather factors. However, weather factors that influence the daily infection risk of cucurbits by P. cubensis have not been determined. Field experiments were conducted from 2012 to 2014 in Clayton, North Carolina and Charleston, South Carolina under naturally occurring inoculum. Weather factors were monitored during the study during which bioassay cucumber plants were exposed to prevailing weather conditions over a 24- and 48-h period. Disease severity was assessed 7 days after exposure and logistic regression models were developed to predict the probability of disease development on the bioassay plants. Hours of relative humidity >80% and day temperature were identified to be important predictors of the risk of infection for both the 24-h and 48-h cases. Threshold probability (PT) on the receiver operating characteristic curve (ROC) that minimized the overall error rate for the 24-h and 48-h model was 0.54 and 0.62, respectively. Model accuracy as estimated using area under ROC (AUC) ranged from 0.82 to 0.86, with a correct classification rate ranging from 0.73 to 0.80. Specificity rates of the models ranged from 0.72 to 0.86, while the sensitivity rates ranged from 0.71 to 0.76. External validation of the models using an independent dataset collected in South Carolina, showed good performance of the models with an AUC >0.91. Using estimates of economic damage and costs of management, optimal decision thresholds that minimized the average costs due to disease control and crop loss were determined. Optimal decision thresholds were dependent on disease prevalence and management decisions informed by the models reduced average costs when disease prevalence was ≤0.50 and ≤0.90 for the 24- and 48-h models, respectively. However, the value of using these models for making disease management decisions was greatest at low levels of disease prevalence. The PT for the validation models was 0.2 and under this threshold, the 24-h model had a correct classification, sensitivity and specificity rates of 0.88, 0.91 and 0.71, respectively, and the corresponding values for the 48-h model were 0.98, 0.97 and 1.0, respectively. These models provide accurate estimates of the daily infection risk of cucumber by P. cubensis and could serve as a useful decision support tool to guide fungicide applications to manage CDM during the growing season.}, journal={Microbial Risk Analysis}, publisher={Elsevier BV}, author={Neufeld, K.N. and Keinath, A.P. and Ojiambo, P.S.}, year={2017}, month={Aug}, pages={21–30} }
 @article{ojiambo_ngugi_scherm_2017, title={CHAPTER 16: Survival Analysis of Time-to-Event Data in Botanical Epidemiology}, DOI={10.1094/9780890544426.016}, abstractNote={HomeEpidemiologyExercises in Plant Disease Epidemiology, Second EditionCHAPTER 16: Survival Analysis of Time-to-Event Data in Botanical Epidemiology PreviousNext CHAPTER 16: Survival Analysis of Time-to-Event Data in Botanical EpidemiologyPeter S. Ojiambo, Henry K. Ngugi, and Harald SchermPeter S. OjiamboSearch for more papers by this author, Henry K. NgugiSearch for more papers by this author, and Harald SchermSearch for more papers by this authorAffiliationsAuthors and Affiliations Published Online:2 Aug 2017https://doi.org/10.1094/9780890544426.016AboutSectionsPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmailWechat Abstract The objectives of this chapter are to acquaint plant pathologists with basic skills in survival analysis techniques. Specifically, we describe (i) types of data that can be analyzed most effectively with survival analysis, (ii) life-table estimates for survivor and hazard functions, (iii) comparison of survivor functions, and (iv) modeling survival data with proportional hazards and accelerated failure time models (including methods for evaluating model fit). DetailsFiguresLiterature CitedRelated Exercises in Plant Disease Epidemiology, Second EditionISBN:978-0-89054-442-6 Metrics Pages: 121-127 InformationPDF download}, journal={Exercises in Plant Disease Epidemiology, Second Edition}, publisher={The American Phytopathological Society}, year={2017}, month={Aug}, pages={121–127} }
 @article{ps_j_bosch f_lv_2017, title={Epidemiology: Past, Present, and Future Impacts on Understanding Disease Dynamics and Improving Plant Disease Management-A Summary of Focus Issue Articles.}, volume={10}, url={http://europepmc.org/abstract/med/29205105}, DOI={10.1094/phyto-07-17-0248-fi}, abstractNote={ Epidemiology has made significant contributions to plant pathology by elucidating the general principles underlying the development of disease epidemics. This has resulted in a greatly improved theoretical and empirical understanding of the dynamics of disease epidemics in time and space, predictions of disease outbreaks or the need for disease control in real-time basis, and tactical and strategic solutions to disease problems. Availability of high-resolution experimental data at multiple temporal and spatial scales has now provided a platform to test and validate theories on the spread of diseases at a wide range of spatial scales ranging from the local to the landscape level. Relatively new approaches in plant disease epidemiology, ranging from network to information theory, coupled with the availability of large-scale datasets and the rapid development of computer technology, are leading to revolutionary thinking about epidemics that can result in considerable improvement of strategic and tactical decision making in the control and management of plant diseases. Methods that were previously restricted to topics such as population biology or evolution are now being employed in epidemiology to enable a better understanding of the forces that drive the development of plant disease epidemics in space and time. This Focus Issue of Phytopathology features research articles that address broad themes in epidemiology including social and political consequences of disease epidemics, decision theory and support, pathogen dispersal and disease spread, disease assessment and pathogen biology and disease resistance. It is important to emphasize that these articles are just a sample of the types of research projects that are relevant to epidemiology. Below, we provide a succinct summary of the articles that are published in this Focus Issue . }, journal={Phytopathology}, author={PS, Ojiambo and J, Yuen and Bosch F and LV, Madden}, year={2017}, month={Sep} }
 @article{ojiambo_yuen_bosch_madden_2017, title={Epidemiology: Past, present, and future impacts on understanding disease dynamics and improving plant disease management-a summary of focus issue articles}, volume={107}, number={10}, journal={Phytopathology}, author={Ojiambo, P. S. and Yuen, J. and Bosch, F. and Madden, L. V.}, year={2017}, pages={1092–1094} }
 @article{ojiambo_gent_mehra_christie_magarey_2017, title={Focus expansion and stability of the spread parameter estimate of the power law model for dispersal gradients}, volume={5}, ISSN={["2167-8359"]}, url={http://europepmc.org/abstract/med/28649473}, DOI={10.7717/peerj.3465}, abstractNote={Empirical and mechanistic modeling indicate that pathogens transmitted via aerially dispersed inoculum follow a power law, resulting in dispersive epidemic waves. The spread parameter (b) of the power law model, which is an indicator of the distance of the epidemic wave front from an initial focus per unit time, has been found to be approximately 2 for several animal and plant diseases over a wide range of spatial scales under conditions favorable for disease spread. Although disease spread and epidemic expansion can be influenced by several factors, the stability of the parameter b over multiple epidemic years has not been determined. Additionally, the size of the initial epidemic area is expected to be strongly related to the final epidemic extent for epidemics, but the stability of this relationship is also not well established. Here, empirical data of cucurbit downy mildew epidemics collected from 2008 to 2014 were analyzed using a spatio-temporal model of disease spread that incorporates logistic growth in time with a power law function for dispersal. Final epidemic extent ranged from 4.16 ×108 km2 in 2012 to 6.44 ×108 km2 in 2009. Current epidemic extent became significantly associated (P < 0.0332; 0.56 < R2 < 0.99) with final epidemic area beginning near the end of April, with the association increasing monotonically to 1.0 by the end of the epidemic season in July. The position of the epidemic wave-front became exponentially more distant with time, and epidemic velocity increased linearly with distance. Slopes from the temporal and spatial regression models varied with about a 2.5-fold range across epidemic years. Estimates of b varied substantially ranging from 1.51 to 4.16 across epidemic years. We observed a significant b ×time (or distance) interaction (P < 0.05) for epidemic years where data were well described by the power law model. These results suggest that the spread parameter b may not be stable over multiple epidemic years. However, b ≈ 2 may be considered the lower limit of the distance traveled by epidemic wave-fronts for aerially transmitted pathogens that follow a power law dispersal function.}, journal={PEERJ}, author={Ojiambo, Peter S. and Gent, David H. and Mehra, Lucky K. and Christie, David and Magarey, Roger}, year={2017}, month={Jun} }
 @article{thomas_carbone_cohen_ojiambo_2017, title={Occurrence and Distribution of Mating Types of Pseudoperonospora cubensis in the United States}, volume={107}, ISSN={0031-949X}, url={http://dx.doi.org/10.1094/PHYTO-06-16-0236-R}, DOI={10.1094/phyto-06-16-0236-r}, abstractNote={ During the past two decades, a resurgence of cucurbit downy mildew has occurred around the world, resulting in severe disease epidemics. In the United States, resurgence of the disease occurred in 2004 and several hypotheses, including introduction of a new genetic recombinant or pathotype of the pathogen, have been suggested as potential causes for this resurgence. Occurrence and distribution of mating types of Pseudoperonospora cubensis in the United States were investigated using 40 isolates collected from cucurbits across 11 states from 2005 to 2013. Pairing of unknown isolates with known mating-type tester strains on detached leaves of cantaloupe or cucumber resulted in oospore formation 8 to 10 days after inoculation. Isolates differed in their ability to form oospores across all coinoculation pairings, with oospore numbers ranging from 280 to 1,000 oospores/cm2 of leaf tissue. Oospores were hyaline to golden-yellow, spherical, and approximately 36 μm in diameter. Of the 40 isolates tested, 24 were found to be of the A1 mating type, while 16 were of the A2 mating type. Mating type was significantly (P < 0.0001) associated with host type, whereby all isolates collected from cucumber were of the A1 mating type, while isolates from squash and watermelon were of the A2 mating type. Similarly, mating type was significantly (P = 0.0287) associated with geographical region, where isolates from northern-tier states of Michigan, New Jersey, New York, and Ohio were all A1, while isolates belonging to either A1 or A2 mating type were present in equal proportions in southern-tier states of Alabama, Florida, Georgia, North Carolina, South Carolina, and Texas. Viability assays showed that oospores were viable and, on average, approximately 40% of the oospores produced were viable as determined by the plasmolysis method. This study showed that A1 and A2 mating types of P. cubensis are present and the pathogen could potentially reproduce sexually in cucurbits within the United States. In addition, the production of viable oospores reported in this study suggests that oospores could have an important role in the biology of P. cubensis and could potentially influence the epidemiology of cucurbit downy mildew in the United States. }, number={3}, journal={Phytopathology}, publisher={Scientific Societies}, author={Thomas, Anna and Carbone, Ignazio and Cohen, Yigal and Ojiambo, Peter S.}, year={2017}, month={Mar}, pages={313–321} }
 @article{thomas_carbone_choe_quesada-ocampo_ojiambo_2017, title={Resurgence of cucurbit downy mildew in the United States: Insights from comparative genomic analysis of Pseudoperonospora cubensis}, volume={7}, ISSN={2045-7758}, url={http://dx.doi.org/10.1002/ECE3.3194}, DOI={10.1002/ece3.3194}, abstractNote={Abstract}, number={16}, journal={Ecology and Evolution}, publisher={Wiley}, author={Thomas, Anna and Carbone, Ignazio and Choe, Kisurb and Quesada-Ocampo, Lina M. and Ojiambo, Peter S.}, year={2017}, month={Jul}, pages={6231–6246} }
 @article{duckworth_andrews_cubeta_grunden_ojiambo_2017, title={Revisiting Graduate Student Training to Address Agricultural and Environmental Societal Challenges}, volume={2}, ISSN={2471-9625}, url={http://dx.doi.org/10.2134/ael2017.06.0019}, DOI={10.2134/ael2017.06.0019}, abstractNote={Core Ideas
Society is faced with daunting environmental and agricultural challenges.
There is a pressing need for multidisciplinary teams of collaborative scientists.
Novel graduate educational models may be needed to train students to address grand challenges.
An example of illustrating the model through microbiome science of plants and soil is presented.
}, number={1}, journal={Agricultural & Environmental Letters}, publisher={Wiley}, author={Duckworth, Owen W. and Andrews, Megan Y. and Cubeta, Marc A. and Grunden, Amy M. and Ojiambo, Peter S.}, year={2017}, pages={170019} }
 @article{thomas_carbone_lebeda_ojiambo_2017, title={Virulence Structure Within Populations of Pseudoperonospora cubensis in the United States}, volume={107}, ISSN={["1943-7684"]}, DOI={10.1094/phyto-07-16-0277-r}, abstractNote={ Cucurbit downy mildew (CDM), caused by the obligate oomycete Pseudoperonospora cubensis, has resurged around the world during the past three decades. A new pathotype or genetic recombinant of P. cubensis have been suggested as possible reasons for the resurgence of CDM in the United States in 2004. In total, 22 isolates collected between 2004 and 2014, mainly in the eastern United States, were tested for their compatibility with a set of 15 cucurbit host types. The virulence structure within these isolates was evaluated on a set of 12 differential genotypes from eight genera. All isolates were highly compatible with the susceptible cultivar of Cucumis sativus, whereas the least compatibility was observed with Luffa cylindrica and Momordica charantia. Based on the compatibility with the differential host set, five pathotypes (1, 3, 4, 5, and 6) were identified among the 22 isolates examined. Pathotypes 1 and 3 had not been previously described in the United States and isolates of these two new pathotypes were also compatible with ‘Poinsett 76’, a cultivar of C. sativus known to be resistant to CDM prior to 2004. Virulence within the pathogen population was expressed based on virulence factors, virulence phenotypes, and virulence complexity. The number of virulence factors ranged from two to eight, indicating a complex virulence structure, with 77% of the isolates having five to eight virulence factors. Thirteen virulence phenotypes were identified; the mean number of virulence factors per isolate and mean number of virulence factors per virulence phenotype was 5.05 and 5.23, respectively, indicating that complex isolates and phenotypes contributed equally to the complex virulence structure of P. cubensis. Gleason and Shannon indices of diversity were 3.88 and 2.32, respectively, indicating a diverse virulence structure of P. cubensis within the United States population. The diverse virulence and high virulence complexity within the pathogen population indicate that host resistance alone in available cucurbit cultivars will not be effective to control CDM. An integrated approach involving a combination of fungicide application and introduction of cultivars with new resistance genes will be required for effective management of CDM. }, number={6}, journal={PHYTOPATHOLOGY}, publisher={Scientific Societies}, author={Thomas, Anna and Carbone, Ignazio and Lebeda, Ales and Ojiambo, Peter S.}, year={2017}, month={Jun}, pages={777–785} }
 @article{atehnkeng_donner_ojiambo_ikotun_augusto_cotty_bandyopadhyay_2016, title={Environmental distribution and genetic diversity of vegetative compatibility groups determine biocontrol strategies to mitigate aflatoxin contamination of maize by Aspergillus flavus}, volume={9}, ISSN={["1751-7915"]}, DOI={10.1111/1751-7915.12324}, abstractNote={Summary}, number={1}, journal={MICROBIAL BIOTECHNOLOGY}, author={Atehnkeng, Joseph and Donner, Matthias and Ojiambo, Peter S. and Ikotun, Babatunde and Augusto, Joao and Cotty, Peter J. and Bandyopadhyay, Ranajit}, year={2016}, month={Jan}, pages={75–88} }
 @article{mehra_cowger_gross_ojiambo_2016, title={Predicting Pre-planting Risk of Stagonospora nodorum blotch in Winter Wheat Using Machine Learning Models}, volume={7}, DOI={10.3389/fpls.2016.00390}, abstractNote={Pre-planting factors have been associated with the late-season severity of Stagonospora nodorum blotch (SNB), caused by the fungal pathogen Parastagonospora nodorum, in winter wheat (Triticum aestivum). The relative importance of these factors in the risk of SNB has not been determined and this knowledge can facilitate disease management decisions prior to planting of the wheat crop. In this study, we examined the performance of multiple regression (MR) and three machine learning algorithms namely artificial neural networks, categorical and regression trees, and random forests (RF), in predicting the pre-planting risk of SNB in wheat. Pre-planting factors tested as potential predictor variables were cultivar resistance, latitude, longitude, previous crop, seeding rate, seed treatment, tillage type, and wheat residue. Disease severity assessed at the end of the growing season was used as the response variable. The models were developed using 431 disease cases (unique combinations of predictors) collected from 2012 to 2014 and these cases were randomly divided into training, validation, and test datasets. Models were evaluated based on the regression of observed against predicted severity values of SNB, sensitivity-specificity ROC analysis, and the Kappa statistic. A strong relationship was observed between late-season severity of SNB and specific pre-planting factors in which latitude, longitude, wheat residue, and cultivar resistance were the most important predictors. The MR model explained 33% of variability in the data, while machine learning models explained 47 to 79% of the total variability. Similarly, the MR model correctly classified 74% of the disease cases, while machine learning models correctly classified 81 to 83% of these cases. Results show that the RF algorithm, which explained 79% of the variability within the data, was the most accurate in predicting the risk of SNB, with an accuracy rate of 93%. The RF algorithm could allow early assessment of the risk of SNB, facilitating sound disease management decisions prior to planting of wheat.}, journal={Frontiers in Plant Science}, publisher={Frontiers Media SA}, author={Mehra, Lucky K. and Cowger, Christina and Gross, Kevin and Ojiambo, Peter S.}, year={2016}, month={Mar} }
 @article{withers_gongora-castillo_gent_thomas_ojiambo_quesada-ocampo_2016, title={Using Next-Generation Sequencing to Develop Molecular Diagnostics for Pseudoperonospora cubensis, the Cucurbit Downy Mildew Pathogen}, volume={106}, ISSN={["1943-7684"]}, url={http://europepmc.org/abstract/med/27314624}, DOI={10.1094/phyto-10-15-0260-fi}, abstractNote={ Advances in next-generation sequencing (NGS) allow for rapid development of genomics resources needed to generate molecular diagnostics assays for infectious agents. NGS approaches are particularly helpful for organisms that cannot be cultured, such as the downy mildew pathogens, a group of biotrophic obligate oomycetes that infect crops of economic importance. Unlike most downy mildew pathogens that are highly host-specific, Pseudoperonospora cubensis causes disease on a broad range of crops belonging to the family Cucurbitaceae. In this study, we identified candidate diagnostic markers for P. cubensis by comparing NGS data from a diverse panel of P. cubensis and P. humuli isolates, two very closely related oomycete species. P. cubensis isolates from diverse hosts and geographical regions in the United States were selected for sequencing to ensure that candidates were conserved in P. cubensis isolates infecting different cucurbit hosts. Genomic regions unique to and conserved in P. cubensis isolates were identified through bioinformatics. These candidate regions were then validated using PCR against a larger collection of isolates from P. cubensis, P. humuli, and other oomycetes. Overall seven diagnostic markers were found to be specific to P. cubensis. These markers could be used for pathogen diagnostics on infected tissue, or adapted for monitoring airborne inoculum with real-time PCR and spore traps. }, number={10}, journal={PHYTOPATHOLOGY}, publisher={Scientific Societies}, author={Withers, S. and Gongora-Castillo, E. and Gent, D. and Thomas, A. and Ojiambo, P. S. and Quesada-Ocampo, L. M.}, year={2016}, month={Oct}, pages={1105–1116} }
 @misc{ojiambo_gent_quesada-ocampo_hausbeck_holmes_2015, title={Epidemiology and population biology of Pseudoperonospora cubensis: A model system for management of downy mildews}, volume={53}, journal={Annual review of phytopathology, vol 53}, author={Ojiambo, P. S. and Gent, D. H. and Quesada-Ocampo, L. M. and Hausbeck, M. K. and Holmes, G. J.}, year={2015}, pages={223–246} }
 @article{ojiambo_gent_quesada-ocampo_hausbeck_holmes_2015, title={Epidemiology and population biology of Pseudoperonospora cubensis: a model system for management of downy mildews.}, volume={53}, url={http://europepmc.org/abstract/med/26002291}, DOI={10.1146/annurev-phyto-080614-120048}, abstractNote={ The resurgence of cucurbit downy mildew has dramatically influenced production of cucurbits and disease management systems at multiple scales. Long-distance dispersal is a fundamental aspect of epidemic development that influences the timing and extent of outbreaks of cucurbit downy mildew. The dispersal potential of Pseudoperonospora cubensis appears to be limited primarily by sporangia production in source fields and availability of susceptible hosts and less by sporangia survival during transport. Uncertainty remains regarding the role of locally produced inoculum in disease outbreaks, but evidence suggests multiple sources of primary inoculum could be important. Understanding pathogen diversity and population differentiation is a critical aspect of disease management and an active research area. Underpinning advances in our understanding of pathogen biology and disease management has been the research capacity and coordination of stakeholders, scientists, and extension personnel. Concepts and approaches developed in this pathosystem can guide future efforts when responding to incursions of new or reemerging downy mildew pathogens. }, number={1}, journal={Annual review of phytopathology}, publisher={Annual Reviews}, author={Ojiambo, PS and Gent, DH and Quesada-Ocampo, LM and Hausbeck, MK and Holmes, GJ}, year={2015}, month={May}, pages={223–246} }
 @article{rahman_ojiambo_louws_2015, title={Initial Inoculum and Spatial Dispersal of Colletotrichum gloeosporioides, the Causal Agent of Strawberry Anthracnose Crown Rot}, volume={99}, ISSN={["1943-7692"]}, DOI={10.1094/pdis-02-13-0144-re}, abstractNote={ Anthracnose crown rot (ACR), caused by Colletotrichum gloeosporioides, is a serious disease of strawberry (Fragaria × ananassa) in nurseries and fruiting fields in the southeastern United States. This study was conducted to determine the potential of alternative hosts for initial inoculum source and spread that causes ACR in strawberry nurseries. Results indicated that Parthenocissus quinquefolia is a noncultivated host of C. gloeosporioides in North Carolina and may serve as an initial inoculum source for planting material. Sources of inoculum data were complemented with a 2-year study of disease incidence and spread in simulated nursery production experiments. Sixty days after inoculation of the mother plants in the nursery, three different inoculation levels showed a significant positive correlation (r = 0.78, P < 0.004) with the quiescent infection (QI) incidence on the runner or daughter plants at the end of the nursery production cycle. Runner plant counts from different proportion of mother plants' inoculation treatments indicated that runner plant production was negatively and significantly (P < 0.001) affected by C. gloeosporioides. Infected tips used to produce transplants destined for fruit production resulted in 29.3 and 16.8% mortality in plug trays in 2007 and 2008, respectively. Tracking foliar QI incidence that resulted from dispersal of inoculum from an introduced point source in the nursery showed a sharp decline at 1 m and beyond from the inoculation focus. Although the exponential model (R2 = 0.92 to 0.94) had slightly higher coefficients of determination than the modified power law (R2 = 0.89 to 0.90), residual plots indicated that the modified power law model fit the disease gradient data better than the exponential model in both years. Results from our dispersal study indicated that rogueing of infected plants within a 4-m radius of infection foci would reduce the risk of transferring infected runner plants from the nursery to the fruiting field. }, number={1}, journal={PLANT DISEASE}, publisher={Scientific Societies}, author={Rahman, Mahfuzur and Ojiambo, Peter and Louws, Frank}, year={2015}, month={Jan}, pages={80–86} }
 @article{mehra_cowger_weisz_ojiambo_2015, title={Quantifying the Effects of Wheat Residue on Severity of Stagonospora nodorum Blotch and Yield in Winter Wheat}, volume={105}, DOI={10.1094/phyto-03-15-0080-r}, abstractNote={ Stagonospora nodorum blotch (SNB), caused by the fungus Parastagonospora nodorum, is a major disease of wheat (Triticum aestivum). Residue from a previously infected wheat crop can be an important source of initial inoculum, but the effects of infected residue on disease severity and yield have not previously been quantified. Experiments were conducted in Raleigh and Salisbury, North Carolina, in 2012, 2013, and 2014 using the moderately susceptible winter wheat cultivar DG Shirley. In 2014, the highly susceptible cultivar DG 9012 was added to the experiment and the study was conducted at an additional site in Tyner, North Carolina. Four (2012) or six (2013 and 2014) wheat residue treatments were applied in the field in a randomized complete block design with five replicates. Treatments in 2012 were 0, 30, 60, and 90% residue coverage of the soil surface, while 10 and 20% residue treatments were added in 2013 and 2014. Across site-years, disease severity ranged from 0 to 50% and increased nonlinearly (P < 0.05) as residue level increased, with a rapid rise to an upper limit and showing little change in severity above 20 to 30% soil surface coverage. Residue coverage had a significant (P < 0.05) effect on disease severity in all site-years. The effect of residue coverage on yield was only significant (P < 0.05) for DG Shirley at Raleigh and Salisbury in 2012 and for DG 9012 at Salisbury in 2014. Similarly, residue coverage significantly (P < 0.05) affected thousand-kernel weight only of DG 9012 in 2014 at Raleigh and Salisbury. Our results showed that when wheat residue was sparse, small additions to residue density produced greater increases in SNB than when residue was abundant. SNB only led to effects on yield and test weight in the most disease-conducive environments, suggesting that the economic threshold for the disease may be higher than previously assumed and warrants review. }, number={11}, journal={Phytopathology}, publisher={Scientific Societies}, author={Mehra, L. K. and Cowger, C. and Weisz, R. and Ojiambo, P. S.}, year={2015}, month={Nov}, pages={1417–1426} }
 @article{holmes_ojiambo_hausbeck_quesada-ocampo_keinath_2015, title={Resurgence of Cucurbit Downy Mildew in the United States: A Watershed Event for Research and Extension}, volume={99}, ISSN={["1943-7692"]}, DOI={10.1094/pdis-09-14-0990-fe}, abstractNote={ In 2004, an outbreak of cucurbit downy mildew (CDM) caused by the oomycete Pseudoperonospora cubensis (Berk. & M. A. Curtis) Rostovzev resulted in an epidemic that stunned the cucumber (Cucumis sativus L.) industry in the eastern United States. The disease affects all major cucurbit crops, including cucumber, muskmelon, squashes, and watermelon. Although the 2004 epidemic began in North Carolina, the cucumber crop from Florida to the northern growing regions in the United States was devastated, resulting in complete crop loss in several areas. Many cucumber fields were abandoned prior to harvest. The rapid spread of the disease coupled with the failure of fungicide control programs surprised growers, crop consultants, and extension specialists. The epidemic raised several fundamental questions about the potential causes for the resurgence of the disease. Some of these questions revolved around whether the epidemic would recur in subsequent years and the possible roles that changes in the host, pathogen, and environment may have played in the epidemic. }, number={4}, journal={PLANT DISEASE}, publisher={Scientific Societies}, author={Holmes, Gerald and Ojiambo, Peter and Hausbeck, Mary and Quesada-Ocampo, Lina and Keinath, Anthony}, year={2015}, month={Apr}, pages={428–441} }
 @article{cohen_langenberg_wehner_ojiambo_hausbeck_quesada-ocampo_lebeda_sierotzki_gisi_2015, title={Resurgence of Pseudoperonospora cubensis: The Causal Agent of Cucurbit Downy Mildew}, volume={105}, ISSN={["1943-7684"]}, url={http://europepmc.org/abstract/med/25844827}, DOI={10.1094/phyto-11-14-0334-fi}, abstractNote={ The downy mildew pathogen, Pseudoperonospora cubensis, which infects plant species in the family Cucurbitaceae, has undergone major changes during the last decade. Disease severity and epidemics are far more destructive than previously reported, and new genotypes, races, pathotypes, and mating types of the pathogen have been discovered in populations from around the globe as a result of the resurgence of the disease. Consequently, disease control through host plant resistance and fungicide applications has become more complex. This resurgence of P. cubensis offers challenges to scientists in many research areas including pathogen biology, epidemiology and dispersal, population structure and population genetics, host preference, host−pathogen interactions and gene expression, genetic host plant resistance, inheritance of host and fungicide resistance, and chemical disease control. This review serves to summarize the current status of this major pathogen and to guide future management and research efforts within this pathosystem. }, number={7}, journal={PHYTOPATHOLOGY}, publisher={Scientific Societies}, author={Cohen, Yigal and Langenberg, Kyle M. and Wehner, Todd C. and Ojiambo, Peter S. and Hausbeck, Mary and Quesada-Ocampo, Lina M. and Lebeda, Ales and Sierotzki, Helge and Gisi, Ulrich}, year={2015}, month={Jul}, pages={998–1012} }
 @article{atehnkeng_ojiambo_cotty_bandyopadhyay_2014, title={Field efficacy of a mixture of atoxigenic Aspergillus flavus Link: Fr vegetative compatibility groups in preventing aflatoxin contamination in maize (Zea mays L.)}, volume={72}, ISSN={["1090-2112"]}, DOI={10.1016/j.biocontrol.2014.02.009}, abstractNote={Competitive exclusion of aflatoxin producers by endemic atoxigenic strains of Aspergillus flavus is a proven tool for aflatoxin management being adapted for use in Africa. Field efficacy of an experimental formulation consisting of four native atoxigenic strains (La3303, La3304, La3279 and Ka16127) was evaluated on maize in 2007 and 2008 in four agroecological zones in Nigeria. The four atoxigenic strains were individually formulated on sterile sorghum grain and subsequently mixed in equal proportions. The blended product was applied on soil (40 kg/ha), 2–3 weeks before flowering. Grains from treated and untreated fields were analyzed for aflatoxins at harvest and after storage. Proportions of the A. flavus population composed of each of the four applied strains in soil before treatment and in harvested grains were determined using vegetative compatibility analyses. Application of the strain mixture resulted in reduced aflatoxin content and significantly (P < 0.05) increased the combined frequencies of the vegetative compatibility groups (VCGs) of the applied strains recovered from the soil and grain. Aflatoxin reductions of 67–95% were associated with a 74–80% combined incidence of the VCGs of the four atoxigenic strains on the treated crops. The applied atoxigenic strains remained with the crop into storage and reduced postharvest increases in contamination. The results suggest that the evaluated multi-strain formulated product has potential to contribute to reduced aflatoxin contamination in Nigeria. This is the first report of a field evaluation of an endemic strain mixture effective at reducing aflatoxin contamination during crop development.}, journal={BIOLOGICAL CONTROL}, author={Atehnkeng, J. and Ojiambo, P. S. and Cotty, P. J. and Bandyopadhyay, R.}, year={2014}, month={May}, pages={62–70} }
 @article{wallace_adams_ivors_ojiambo_quesada-ocampo_2014, title={First Report of Pseudoperonospora cubensis Causing Downy Mildew on Momordica balsamina and M. charantia in North Carolina}, volume={98}, ISSN={["1943-7692"]}, url={http://europepmc.org/abstract/med/30699625}, DOI={10.1094/pdis-03-14-0305-pdn}, abstractNote={ Momordica balsamina (balsam apple) and M. charantia L. (bitter melon/bitter gourd/balsam pear) commonly grow in the wild in Africa and Asia; bitter melon is also cultivated for food and medicinal purposes in Asia (1). In the United States, these cucurbits grow as weeds or ornamentals. Both species are found in southern states and bitter melon is also found in Pennsylvania and Connecticut (3). Cucurbit downy mildew (CDM), caused by the oomycete Pseudoperonospora cubensis, was observed on bitter melon and balsam apple between August and October of 2013 in six North Carolina sentinel plots belonging to the CDM ipmPIPE program (2). Plots were located at research stations in Johnston, Sampson, Lenoir, Henderson, Rowan, and Haywood counties, and contained six different commercial cucurbit species including cucumbers, melons, and squashes in addition to the Momordica spp. Leaves with symptoms typical of CDM were collected from the Momordica spp. and symptoms varied from irregular chlorotic lesions to circular lesions with chlorotic halos on the adaxial leaf surface. Sporulation on the abaxial side of the leaves was observed and a compound microscope revealed sporangiophores (180 to 200 μm height) bearing lemon-shaped, dark sporangia (20 to 35 × 10 to 20 μm diameter) with papilla on one end. Genomic DNA was extracted from lesions and regions of the NADH dehydrogynase subunit 1 (Nad1), NADH dehydrogynase subunit 5 (Nad5), and internal transcribed spacer (ITS) ribosomal RNA genes were amplified and sequenced (4). BLAST analysis revealed 100% identity to P. cubensis Nad1 (HQ636552.1, HQ636551.1), Nad5 (HQ636556.1), and ITS (HQ636491.1) sequences in GenBank. Sequences from a downy mildew isolate from each Momordica spp. were deposited in GenBank as accession nos. KJ496339 through 44. To further confirm host susceptibility, vein junctions on the abaxial leaf surface of five detached leaves of lab-grown balsam apple and bitter melon were either inoculated with a sporangia suspension (10 μl, 104 sporangia/ml) of a P. cubensis isolate from Cucumis sativus (‘Vlaspik' cucumber), or with water as a control. Inoculated leaves were placed in humidity chambers to promote infection and incubated using a 12-h light (21°C) and dark (18°C) cycle. Seven days post inoculation, CDM symptoms and sporulation were observed on inoculated balsam apple and bitter melon leaves. P. cubensis has been reported as a pathogen of both hosts in Iowa (5). To our knowledge, this is the first report of P. cubensis infecting these Momordica spp. in NC in the field. Identifying these Momordica spp. as hosts for P. cubensis is important since these cucurbits may serve as a source of CDM inoculum and potentially an overwintering mechanism for P. cubensis. Further research is needed to establish the role of non-commercial cucurbits in the yearly CDM epidemic, which will aid the efforts of the CDM ipmPIPE to predict disease outbreaks. }, number={9}, journal={PLANT DISEASE}, publisher={Scientific Societies}, author={Wallace, E. and Adams, M. and Ivors, K. and Ojiambo, P. S. and Quesada-Ocampo, L. M.}, year={2014}, month={Sep}, pages={1279–1279} }
 @article{twizeyimana_ojiambo_bandyopadhyay_hartman_2014, title={Use of Quantitative Traits to Assess Aggressiveness of Phakopsora pachyrhizi Isolates from Nigeria and the United States}, volume={98}, ISSN={["1943-7692"]}, url={http://europepmc.org/abstract/AGR/IND500906801}, DOI={10.1094/pdis-12-13-1247-re}, abstractNote={ Soybean rust, caused by Phakopsora pachyrhizi, is one of the most important foliar diseases of soybean worldwide. The soybean–P. pachyrhizi interaction is often complex due to genetic variability in host and pathogen genotypes. In a compatible reaction, soybean genotypes produce tan-colored lesions, whereas in an incompatible reaction soybean genotypes produce an immune response (complete resistance) or reddish-brown lesions (incomplete resistance). In this study, in total, 116 and 72 isolates of P. pachyrhizi from Nigeria and the United States, respectively, were compared based on six quantitative traits to assess their aggressiveness on two soybean genotypes. All isolates produced reddish-brown lesions on plant introduction (PI) 462312 and tan lesions on TGx 1485-1D. The number of days after inoculation to first appearance of lesions, uredinia, and sporulation, along with the number of lesions and sporulating uredinia per square centimeter of leaf tissue, and the number of uredinia per lesion, were significantly (P < 0.001) different between the two soybean genotypes for all isolates from each country. The number of days to first appearance of lesions, uredinia, and sporulation were greater on PI 462312 than on TGx 1485-1D for all the test isolates. Similarly, the number of lesions and sporulating uredinia per square centimeter, and the number of uredinia per lesion were lower on PI 462312 than on TGx 1485-1D. For both soybean genotypes, the number of sporulating uredinia per square centimeter significantly (P = 0.0001) increased with an increase in the number of lesions per square centimeter. Although the slope of the regression of sporulating uredinia on number of lesions was greater (P < 0.0001) when TGx 1485-1D was inoculated with Nigerian isolates compared with U.S. isolates, slopes of the regression lines did not differ significantly (P > 0.0675) when PI 46312 was inoculated with Nigerian or U.S. isolates. This is the first study that used a large number of isolates from two continents to assess aggressiveness of P. pachyrhizi using multiple traits in soybean genotypes with contrasting types of disease reaction. }, number={9}, journal={PLANT DISEASE}, author={Twizeyimana, M. and Ojiambo, P. S. and Bandyopadhyay, R. and Hartman, G. L.}, year={2014}, month={Sep}, pages={1261–1266} }
 @article{call_wehner_holmes_ojiambo_2013, title={Effects of Host Plant Resistance and Fungicides on Severity of Cucumber Downy Mildew}, volume={48}, ISSN={0018-5345 2327-9834}, url={http://dx.doi.org/10.21273/hortsci.48.1.53}, DOI={10.21273/hortsci.48.1.53}, abstractNote={Cucurbit downy mildew caused by the oomycete Pseudoperonospora cubensis (Berk. And Curt) Rostov is a major disease of cucumber (Cucumis sativus L.) (Palti and Cohen, 1980) globally. Chemical control of downy mildew is necessary to achieve high yields in the absence of adequate host plant resistance. Most of the currently grown cultivars have some resistance to downy mildew. Before the resurgence of the disease in 2004, host plant resistance was sufficient to control the disease without fungicide use, and downy mildew was only a minor problem on cucumber. There are currently no cultivars that show resistance at a level equal to that observed before 2004. However, differences in resistance exist among cultivars, ranging from moderately resistant to highly susceptible. In this study, we evaluated the disease severity and yield of four cucumber cultivars that differed in disease resistance and were treated with fungicide programs representing a range of efficacy levels. The experiment was a split plot design with six replications and four years. Disease was evaluated as chlorosis, necrosis, and reduction in plant size on a 0 to 9 scale. Cultigen had a large effect in all four years. Fungicide has a smaller effect on resistance component traits and a larger effect on yield traits. The effects of cultivar resistance and fungicides appear to be additive until a threshold where maximum yield is reached. Highly resistant cultigens such as PI 197088 required only the least effective fungicides to achieve highest yields, whereas moderately resistant cultigens required a more effective fungicide to reach a similar level of yield. Susceptible cultigens did not achieve high yield even with the most effective fungicide treatments. It is likely that, even as highly resistant cultivars are released, growers will need to continue a minimal fungicide program to achieve maximum yield.}, number={1}, journal={HortScience}, publisher={American Society for Horticultural Science}, author={Call, Adam D. and Wehner, Todd C. and Holmes, Gerald J. and Ojiambo, Peter S.}, year={2013}, month={Jan}, pages={53–59} }
 @article{ojiambo_kang_2013, title={Modeling Spatial Frailties in Survival Analysis of Cucurbit Downy Mildew Epidemics}, volume={103}, ISSN={["1943-7684"]}, DOI={10.1094/phyto-07-12-0152-r}, abstractNote={ Cucurbit downy mildew caused by Pseudoperonospora cubensis is economically the most important disease of cucurbits globally, and the pathogen is disseminated aerially over a large spatial scale. Spatio-temporal spread of the disease was characterized during phase I (low and sporadic disease outbreaks) and II (rapid increase in disease outbreaks) of the epidemic using records collected from sentinel plots from 2008 to 2009 in 23 states in the eastern United States as part of the United States Department of Agriculture Cucurbit Downy Mildew ipmPIPE network. A substantive goal of this study was to explain the pattern of time to disease outbreak using important covariates while accounting for spatially correlated differences in risk of disease outbreak among the states. Survival analyses that accounts for spatial dependence were performed on time to disease outbreak, and posterior median frailties (or random effects) were mapped to identify states with high or low risk for disease outbreak. From February to October, disease occurred in 195 and 172 out of 413 and 556 cases monitored in 2008 and 2009, respectively. Disease outbreaks were spatially aggregated, with a spatial dependence of up to ≈1,025 km where clustering of outbreaks in phase I and II of the epidemic were similar. However, unlike in phase I of the epidemic, space–time point pattern analysis was significant (P < 0.0001) for outbreaks in phase II, during which the highest risk window as estimated by the space–time function was within 1.5 months and 500 km of the initial outbreak. The risk of disease outbreak peaked around July and decreased thereafter until the end of the study period. Spatially correlated analysis of time to disease outbreak indicated the need to incorporate spatial frailties in standard survival analysis models. Evaluation of alternative formulations of the spatial models demonstrated that a Bayesian hierarchical spatially structured frailty model best described time to disease outbreak. This frailty model showed clustering of outbreaks at the state level and indicated that states in the mid-Atlantic region have high spatial frailties and a high risk of downy mildew outbreak. }, number={3}, journal={PHYTOPATHOLOGY}, publisher={Scientific Societies}, author={Ojiambo, P. S. and Kang, E. L.}, year={2013}, month={Mar}, pages={216–227} }
 @article{zandjanakou-tachin_ojiambo_vroh-bi_tenkouano_gumedzoe_bandyopadhyay_2013, title={Pathogenic variation of Mycosphaerella species infecting banana and plantain in Nigeria}, volume={62}, ISSN={["0032-0862"]}, url={http://europepmc.org/abstract/AGR/IND500644009}, DOI={10.1111/j.1365-3059.2012.02650.x}, abstractNote={Mycosphaerella species that cause the ‘Sigatoka disease complex’ account for significant yield losses in banana and plantain worldwide. Disease surveys were conducted in the humid forest (HF) and derived savanna (DS) agroecological zones from 2004 to 2006 to determine the distribution of the disease and variation among Mycosphaerella species in Nigeria. Disease prevalence and severity were higher in the HF than in the DS zone, but significant (P < 0·001) differences between agroecological zones were only observed for disease severity. A total of 85 isolates of M. fijiensis and 11 isolates of M. eumusae were collected during the survey and used to characterize the pathogenic structure of Mycosphaerella spp. using a putative host differential cultivar set consisting of Calcutta‐4 (resistant), Valery (intermediate) and Agbagba (highly susceptible). Area under disease progress curve (AUDPC) was higher on all cultivars when inoculated with M. eumusae than with M. fijiensis, but significant (P < 0·05) differences between the two species were only observed on Valery. Based on the rank‐sum method, 8·3% of the isolates were classified as highly aggressive and 46·9% were classified as aggressive. About 11·5% of all the isolates were classified as least aggressive, and all of these were M. fijiensis. The majority of M. eumusae isolates (seven out of 11; 64%) were classified as aggressive. A total of nine pathotype clusters were identified using cluster analysis of AUDPC. At least one M. fijiensis isolate was present in all the nine pathotype clusters, while isolates of M. eumusae were present in six of the nine clusters. Isolates in pathotype clusters III and V were the most aggressive, while those in cluster VIII were the least aggressive. Shannon’s index (H) revealed a more diverse Mycosphaerella collection in the DS zone (H = 1·81) than in the HF (H = 1·50) zone, with M. fijiensis being more diverse than M. eumusae. These results describe the current pathotype structure of Mycosphaerella in Nigeria and provide a useful resource that will facilitate screening of newly developed Musa genotypes for resistance against two important leaf spot diseases of banana and plantain.}, number={2}, journal={PLANT PATHOLOGY}, author={Zandjanakou-Tachin, M. and Ojiambo, P. S. and Vroh-Bi, I. and Tenkouano, A. and Gumedzoe, Y. M. and Bandyopadhyay, R.}, year={2013}, month={Apr}, pages={298–308} }
 @article{neufeld_isard_ojiambo_2013, title={Relationship between disease severity and escape of Pseudoperonospora cubensis sporangia from a cucumber canopy during downy mildew epidemics}, volume={62}, ISSN={["1365-3059"]}, DOI={10.1111/ppa.12040}, abstractNote={Fundamental to the development of models to predict the spread of cucurbit downy mildew is the ability to determine the escape of Pseudoperonospora cubensis sporangia from infected fields. Aerial concentrations of sporangia, C (sporangia m−3), were monitored using Rotorod samplers deployed at 0·5 to 3·0 m above a naturally infected cucumber canopy in two sites in central and eastern North Carolina in 2011, where disease severity ranged from 1 to 40%. Standing crop of sporangia was assessed each morning at 07·00 h EDT and ranged from 320 to 16 170 sporangia m−2. Disease severity and height above the canopy significantly (P < 0·0001) affected C with mean concentration (Cm) being high at moderate disease. Values of Cm decreased rapidly with canopy height and at a height of 2·0 m, Cm was only 7% of values measured at 0·5 m when disease was moderate. Daily total flux (FD) was dependent on disease severity and ranged from 5·9 to 2242·3 sporangia m−2. The fraction of available sporangia that escaped the canopy increased from 0·028 to 0·171 as average wind speed above the canopy for periods of high C increased from 1·7 to 3·6 m s−1. Variations of Cm and FD with increasing disease were well described (P < 0·0001) by a log‐normal model with 15% as the threshold above which Cm and FD decreased as disease severity increased. These results indicate that disease severity should be used to adjust sporangia escape in spore transport simulation models that are used to predict the risk of spread of cucurbit downy mildew.}, number={6}, journal={PLANT PATHOLOGY}, author={Neufeld, K. N. and Isard, S. A. and Ojiambo, P. S.}, year={2013}, month={Dec}, pages={1366–1377} }
 @article{neufeld_ojiambo_2012, title={Interactive Effects of Temperature and Leaf Wetness Duration on Sporangia Germination and Infection of Cucurbit Hosts by Pseudoperonospora cubensis}, volume={96}, DOI={10.1094/pdis-07-11-0560}, abstractNote={ Outbreaks of cucurbit downy mildew caused by Pseudoperonospora cubensis are dependent on the weather but effects of temperature and leaf wetness duration on infection have not been studied for different cucurbits. To determine the effects of these two weather variables on sporangia germination and infection of cucurbit host types by P. cubensis, three host types; cucumber (‘Straight 8’), cantaloupe (‘Kermit’), and acorn squash (‘Table Queen’), were inoculated and exposed to leaf wetness durations of 2 to 24 h at six constant temperatures ranging from 5 to 30°C in growth-chamber experiments. Sporangia germination was assessed after each wetness period, and leaf area infected was assessed 5 and 7 days after inoculation. Germination of sporangia was highest on cantaloupe (16.5 to 85.7%) and lowest on squash (10.7 to 68.9%), while disease severity was highest and lowest on cucumber and cantaloupe, respectively. Host type, temperature, wetness duration and their interactions significantly (P < 0.0001) affected germination and disease severity. Germination and disease data for each host type were separately fitted to a modified form of a Weibull function that characterizes a unimodal response and monotonic increase of germination or infection with temperature and wetness duration, respectively. The effect of host type on germination and infection was characterized primarily by differences in the upper limit parameter in response to temperature. Differences among host types based on other parameters were either small or inconsistent. Temperature and wetness duration that supported a given level of germination or infection varied among host types. At 20°C, 15% leaf area infected was expected following 2, 4, and 8 h of wetness for cucumber, squash, and cantaloupe, respectively. When temperature was increased to 25°C, 15% disease severity was expected following 3, 7, and 15 h of wetness for cucumber, squash, and cantaloupe, respectively. Risk charts were constructed to estimate the potential risk of infection of cucurbit host types by P. cubensis based on prevailing or forecasted temperature and leaf wetness duration. These results will improve the timing and application of the initial fungicide spray for the control of cucurbit downy mildew. }, number={3}, journal={Plant Disease}, publisher={Scientific Societies}, author={Neufeld, K. N. and Ojiambo, P. S.}, year={2012}, month={Mar}, pages={345–353} }
 @article{crook_friesen_liu_ojiambo_cowger_2012, title={Novel Necrotrophic Effectors from Stagonospora nodorum and Corresponding Host Sensitivities in Winter Wheat Germplasm in the Southeastern United States}, volume={102}, ISSN={["1943-7684"]}, url={http://europepmc.org/abstract/med/22494247}, DOI={10.1094/phyto-08-11-0238}, abstractNote={ Stagonospora nodorum blotch (SNB), caused by the necrotrophic fungus Stagonospora nodorum (teleomorph: Phaeosphaeria nodorum), is among the most common diseases of winter wheat in the United States. New opportunities in resistance breeding have arisen from the recent discovery of several necrotrophic effectors (NEs, also known as host-selective toxins) produced by S. nodorum, along with their corresponding host sensitivity (Snn) genes. Thirty-nine isolates of S. nodorum collected from wheat debris or grain from seven states in the southeastern United States were used to investigate the production of NEs in the region. Twenty-nine cultivars with varying levels of resistance to SNB, representing 10 eastern-U.S. breeding programs, were infiltrated with culture filtrates from the S. nodorum isolates in a randomized complete block design. Three single-NE Pichia pastoris controls, two S. nodorum isolate controls, and six Snn-differential wheat controls were also used. Cultivar–isolate interactions were visually evaluated for sensitivity at 7 days after infiltration. Production of NEs was detected in isolates originating in each sampled state except Maryland. Of the 39 isolates, 17 produced NEs different from those previously characterized in the upper Great Plains region. These novel NEs likely correspond to unidentified Snn genes in Southeastern wheat cultivars, because NEs are thought to arise under selection pressure from genes for resistance to biotrophic pathogens of wheat cultivars that differ by geographic region. Only 3, 0, and 23% of the 39 isolates produced SnToxA, SnTox1, and SnTox3, respectively, by the culture-filtrate test. A Southern dot-blot test showed that 15, 74, and 39% of the isolates carried the genes for those NEs, respectively; those percentages were lower than those found previously in larger international samples. Only two cultivars appeared to contain known Snn genes, although half of the cultivars displayed sensitivity to culture filtrates containing unknown NEs. Effector sensitivity was more frequent in SNB-susceptible cultivars than in moderately resistant (MR) cultivars (P = 0.008), although some susceptible cultivars did not exhibit sensitivity to NEs produced by isolates in this study and some MR cultivars were sensitive to NEs of multiple isolates. Our results suggest that NE sensitivities influence but may not be the only determinant of cultivar resistance to S. nodorum. Specific knowledge of NE and Snn gene frequencies in this region can be used by wheat breeding programs to improve SNB resistance. }, number={5}, journal={PHYTOPATHOLOGY}, author={Crook, A. D. and Friesen, T. L. and Liu, Z. H. and Ojiambo, P. S. and Cowger, C.}, year={2012}, month={May}, pages={498–505} }
 @article{olanya_he_larkin_honeycutt_nyankanga_ojiambo_lemaga_kakuhenzire_fontem_2012, place={Springer Netherlands, Dordrecht}, title={Optimization of Late Blight and Bacterial Wilt Management in Potato Production Systems in the Highland Tropics of Africa}, author={Olanya, M. and He, Z. and Larkin, R. and Honeycutt, W. and Nyankanga, R. and Ojiambo, P. and Lemaga, B. and Kakuhenzire, R. and Fontem, D.}, year={2012}, month={Apr} }
 @article{zandjanakou-tachin_ojiambo_vroh-bi_tenkouano_gumedzoe_bandyopadhyay_2012, title={Pathogenic variation of Mycosphaerella species infecting banana and plantain in Nigeria}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84863100338&partnerID=MN8TOARS}, journal={Plant Pathology}, author={Zandjanakou-Tachin, M. and Ojiambo, P.S. and Vroh-Bi, I. and Tenkouano, A. and Gumedzoe, Y.M. and Bandyopadhyay, R.}, year={2012} }
 @article{zeatfoss_cowger_ojiambo_2011, title={A Degree-Day Model for the Latent Period of Stagonospora nodorum Blotch in Winter Wheat}, volume={95}, ISSN={["0191-2917"]}, DOI={10.1094/pdis-09-10-0651}, abstractNote={ Stagonospora nodorum blotch (SNB), which is caused by Stagonospora nodorum, occurs frequently in the southeastern United States, and severe epidemics can lead to substantial yield losses. To develop a model for the progress of SNB based on the effects of temperature on the latent period of the pathogen, batches of two winter wheat cultivars, AGS 2000 and USG 3209, were inoculated with S. nodorum at weekly intervals for 16 weeks. After 72 h of incubation, inoculated plants were exposed to outdoor conditions where temperatures ranged from –6.6 to 35.8°C, with a mean batch temperature ranging from 9.7 to 24.7°C. Latent period, expressed as time from inoculation until the first visible lesions with pycnidia, ranged from 13 to 34 days. The relationship between the inverse of the latent period and mean temperature was best described by a linear model, and the estimated thermal time required for the completion of the latent period was 384.6 degree-days. A shifted cumulative gamma distribution model with a base temperature of 0.5°C significantly (P < 0.0001) described the relationship between increasing number of lesions with pycnidia and accumulated thermal time. When latent period was defined as time to 50% of the maximum number of lesions with pycnidia (L50), the model estimated L50 as 336 and 326 degree-days above 0.5°C for AGS 2000 and USG 3209, respectively. The relationship between 1/L50 and mean temperature was also best described using a linear model (r2 = 0.93, P < 0.001). This study provides data that link disease progress with wheat growth, which facilitates accurate identification of thresholds for timing of fungicide applications. }, number={5}, journal={PLANT DISEASE}, publisher={Scientific Societies}, author={Zeatfoss, A. D. and Cowger, C. and Ojiambo, P. S.}, year={2011}, month={May}, pages={561–567} }
 @article{zearfoss_cowger_ojiambo_2011, title={A degree-day model for the latent period of Stagonospora nodorum blotch in winter wheat}, volume={95}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79955396485&partnerID=MN8TOARS}, number={5}, journal={Plant Disease}, author={Zearfoss, A.D. and Cowger, C. and Ojiambo, P.S.}, year={2011}, pages={561–567} }
 @article{diedhiou_bandyopadhyay_atehnkeng_ojiambo_2011, title={Aspergillus Colonization and Aflatoxin Contamination of Maize and Sesame Kernels in Two Agro-ecological Zones in Senegal}, volume={159}, ISSN={["0931-1785"]}, url={http://europepmc.org/abstract/AGR/IND44501903}, DOI={10.1111/j.1439-0434.2010.01761.x}, abstractNote={Aflatoxin contamination of major food crops is a serious problem in Senegal. Maize and sesame samples were collected during a survey in five districts located in two agro-ecological zones in Senegal to determine levels of aflatoxin contamination and the distribution and toxigenicity potential of members of Aspergillus section Flavi. Maize samples from the Guinea Savannah zone (SG) exhibited lower aflatoxin content and colony-forming units (cfu) than those collected from the Sudan Savannah (SS) zone. In maize, aflatoxin concentration and cfu of A.flavus varied with cultivars, shelling practices and storage methods. The maize variety 'Jaune de Bambey' had high aflatoxin levels in both agro-ecological zones. Aflatoxin content in machine-shelled maize (120 ng/g) was more than 10-fold higher than that in manually shelled (8 ng/g) or unshelled maize. Aflatoxin content (between 0.1 and 1.2 ng/g) and cfu values (between 13 and 42 000 cfu/g) of sesame were low, suggesting a low susceptibility to A. flavus. In both agro-ecological zones, and in all storage systems, aflatoxin contamination was lower in sesame than in maize. In this study, only three species of Aspergillus section Flavi (A. flavus, A. tamarii and the unnamed taxon S BG ) were observed with the frequency of toxigenic strains remaining below 50% in maize from the SG zone compared with 51% of isolates from samples collected in Sedhiou district in SS zone. The proportion of toxigenic strains isolated from sesame was variable. For both crops, L-strains were the most prevalent in the two agro-ecological zones. Some of the atoxigenic strains collected could be valuable microbial resources for the biological control of aflatoxin in Senegal.}, number={4}, journal={JOURNAL OF PHYTOPATHOLOGY}, author={Diedhiou, Papa Madiallacke and Bandyopadhyay, Ranajit and Atehnkeng, Joseph and Ojiambo, Peter S.}, year={2011}, month={Apr}, pages={268–275} }
 @article{twizeyimana_ojiambo_hartman_bandyopadhyay_2011, title={Dynamics of Soybean Rust Epidemics in Sequential Plantings of Soybean Cultivars in Nigeria}, volume={95}, ISSN={["1943-7692"]}, url={http://europepmc.org/abstract/AGR/IND44469662}, DOI={10.1094/pdis-06-10-0436}, abstractNote={ Soybean rust, caused by Phakopsora pachyrhizi, is an important foliar disease of soybean. Disease severity is dependent on several environmental factors, although the precise nature of most of these factors under field conditions is not known. To help understand the environmental factors that affect disease development, soybean rust epidemics were studied in Nigeria by sequentially planting an early-maturing, highly susceptible cultivar, TGx 1485-1D, and a late-maturing, moderately susceptible cultivar, TGx 1448-2E, at 30- to 45-day intervals from August 2004 to September 2006. Within each planting date, disease onset occurred earlier on TGx 1485-1D than on TGx 1448-2E, and rust onset was at least 20 days earlier on soybean planted between August and October than on soybean planted between November and April. The logistic model provided a better description of the temporal increase in rust severity than the Gompertz model. Based on the logistic model, the highest absolute rates of disease increase were observed on soybean planted in September 2006 and October 2004 for TGx 1485-1D and TGx 1448-2E, respectively. Disease severity as measured by the relative area under disease progress curve (RAUDPC) was significantly (P < 0.05) negatively correlated with evaporation (r = –0.73), solar radiation (r = –0.59), and temperature (r = –0.64) but positively correlated with urediniospore concentration (r = 0.58). Planting date and soybean cultivar significantly (P < 0.05) affected disease severity, with severity being higher on soybean crops planted during the wet season than those planted in the dry season. Within the wet season, planting in May and July resulted in a significantly (P < 0.05) lower RAUDPC than planting between August and October. Yields were significantly (P < 0.001) related to RAUDPC during the wet season, whereby an increase in RAUDPC resulted in a linear decrease in yield. This study suggests that selection of planting date could be a useful cultural practice for reducing soybean rust. }, number={1}, journal={PLANT DISEASE}, author={Twizeyimana, M. and Ojiambo, P. S. and Hartman, G. L. and Bandyopadhyay, R.}, year={2011}, month={Jan}, pages={43–50} }
 @article{twizeyimana_ojiambo_hartmand_bandyopadhyay_2011, title={Dynamics of soybean rust epidemics in sequential plantings of soybean cultivars in Nigeria}, volume={95}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79751512238&partnerID=MN8TOARS}, number={1}, journal={Plant Disease}, author={Twizeyimana, M. and Ojiambo, P.S. and Hartmand, G.L. and Bandyopadhyay, R.}, year={2011}, pages={43–50} }
 @article{twizeyimana_ojiambo_haudenshield_caetano-anolles_pedley_bandyopadhyay_hartman_2011, title={Genetic structure and diversity of Phakopsora pachyrhizi isolates from soyabean}, volume={60}, ISSN={["1365-3059"]}, url={http://europepmc.org/abstract/AGR/IND44598231}, DOI={10.1111/j.1365-3059.2011.02428.x}, abstractNote={Simple sequence repeat (SSR) markers were used to classify 116 isolates of Phakopsora pachyrhizi, the cause of soyabean rust, collected from infected soyabean leaves in four agroecological zones in Nigeria. A high degree of genetic variation was observed within the sampled populations of P. pachyrhizi. Eighty‐four distinct genotypes were identified among three of the four agroecological zones. Nei’s average genetic diversity across geographical regions was 0·22. Hierarchical analysis of molecular variance showed low genetic differentiation among all populations of P. pachyrhizi. The majority (> 90%) of the genetic diversity was distributed within each soyabean field, while approximately 6% of the genetic diversity was distributed among fields within geographic regions. Low population differentiation was indicated by the low FST values among populations, suggesting a wide dispersal of identical genotypes on a regional scale. Phylogenetic analysis indicated a strictly clonal structure of the populations and five main groups were observed, with group II accounting for 30% of the entire population. Because of the asexual reproduction of P. pachyrhizi, single‐step mutations in SSR genotypes are likely to account for the genetic differences within each group.}, number={4}, journal={PLANT PATHOLOGY}, author={Twizeyimana, M. and Ojiambo, P. S. and Haudenshield, J. S. and Caetano-Anolles, G. and Pedley, K. F. and Bandyopadhyay, R. and Hartman, G. L.}, year={2011}, month={Aug}, pages={719–729} }
 @article{ojiambo_holmes_2011, title={Spatiotemporal Spread of Cucurbit Downy Mildew in the Eastern United States}, volume={101}, ISSN={["1943-7684"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79954559002&partnerID=MN8TOARS}, DOI={10.1094/phyto-09-10-0240}, abstractNote={ The dynamics of cucurbit downy mildew, caused by Pseudoperonospora cubensis, in the eastern United States in 2008 and 2009 were investigated based on disease records collected in 24 states as part of the Cucurbit downy mildew ipmPIPE monitoring program. The mean season-long rate of temporal disease progress across the 2 years was 1.4 new cases per day. Although cucurbit downy mildew was detected in mid-February and early March in southern Florida, the disease progressed slowly during the spring and early summer and did not enter its exponential phase until mid-June. The median nearest-neighbor distance of spread of new disease cases was ≈110 km in both years, with ≈15% of the distances being >240 km. Considering disease epidemics on all cucurbits, the epidemic expanded at a rate of 9.2 and 10.5 km per day in 2008 and 2009, respectively. These rates of spatial spread are at the lower range of those reported for the annual spread of tobacco blue mold in the southeastern United States, a disease that is also aerially dispersed over long distances. These results suggest that regional spread of cucurbit downy mildew may be limited by opportunities for establishment in the first half of the year, when fewer cucurbit hosts are available for infection. The O-ring statistic was used to determine the spatial pattern of cucurbit downy mildew outbreaks using complete spatial randomness as the null model for hypothesis testing. Disease outbreaks in both years were spatially aggregated and the extent of spatial dependence was up to 1,000 km. Results from the spatial analysis suggests that disease outbreaks in the Great Lakes and mid-Atlantic regions may be due to the spread of P. cubensis sporangia from outbreaks of the disease near the Georgia/South Carolina/North Carolina border rather than from overwintering sites in southern Florida. Space–time point pattern analysis indicated strong (P < 0.001) evidence for a space–time interaction and a space–time risk window of ≈3 to 5 months after first disease outbreak and 300 to 600 km was detected in both years. Results of this study support the hypothesis that infection of cucurbits by P. cubensis appears to be an outcome of a contagion process, and the relative large space–time window suggests that factors occurring on a large spatial scale (≈1,000 km) facilitate the spread of cucurbit downy mildew in the eastern United States. }, number={4}, journal={PHYTOPATHOLOGY}, publisher={Scientific Societies}, author={Ojiambo, P. S. and Holmes, G. J.}, year={2011}, month={Apr}, pages={451–461} }
 @article{nyankanga_olanya_ojiambo_wien_honeycutt_kirk_2011, title={Validation of tuber blight (Phytophthora infestans) prediction model}, volume={30}, ISSN={["1873-6904"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79952359482&partnerID=MN8TOARS}, DOI={10.1016/j.cropro.2010.11.019}, abstractNote={Potato tuber blight caused by Phytophthora infestans accounts for significant losses of tubers in storage. Despite research on infection and management of tuber blight, there is paucity of information on the prediction of the occurrence tuber blight or modelling of tuber infection by P. infestans under field conditions. A tuber blight prediction model was developed in New York in experiments conducted using cultivars Allegany, NY101, and Katahdin in 1998 and 1999. This model was validated using data collected from the potato cultivar Snowden in field experiments in Laingsburg, Michigan from 2000 to 2009. In both New York and Michigan experiments, disease was initiated by artificial inoculation of cultivars with a US-8 isolate of P. infestans. Mean leaf area affected ranged from 0 to 94% at New York, and 0 to 93% at Michigan. At New York and Michigan, mean tuber blight incidences ranged from 1 to 40% and 0 to 15%, respectively. In the validation of the model using data collected at Laingsburg, Michigan, the model correctly predicted tuber blight incidence in 7 out of 9 years. Comparison of observed with predicted values indicated that slopes of the regression line between observed and predicted germination and infection data were not significantly different (P > 0.3547). Correlation coefficient between observed and predicted values was high (r2 > 0.65) and the coefficient of variation of the residuals of error was about 12%. Although inoculum availability is assumed in the model, incorporation of relationships of inoculum density, propagule survival in soil, and tuber blight incidence would greatly improve the prediction of tuber blight under field conditions.}, number={5}, journal={CROP PROTECTION}, publisher={Elsevier BV}, author={Nyankanga, R. O. and Olanya, O. M. and Ojiambo, P. S. and Wien, H. C. and Honeycutt, C. W. and Kirk, W. W.}, year={2011}, month={May}, pages={547–553} }
 @article{ojiambo_paul_holmes_2010, title={A Quantitative Review of Fungicide Efficacy for Managing Downy Mildew in Cucurbits}, volume={100}, ISSN={["1943-7684"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77958058867&partnerID=MN8TOARS}, DOI={10.1094/phyto-12-09-0348}, abstractNote={A meta-analysis of the effect of fungicides on cucurbit downy mildew was conducted using data previously published in Fungicide & Nematicide Tests and Plant Disease Management Reports from 2000 to 2008. Standardized mean effect size (the difference in disease intensity expressed in standard deviation units between the fungicide treatment and its corresponding untreated control) was calculated for each of the 105 field studies evaluating the effects of fungicides on cucurbit downy mildew. Fixed- and random-effects meta-analyses were performed on the log-transformed standardized mean effect sizes to estimate the overall mean effect size ([Formula: see text]), and to determine the variability in the effect size among studies. Fungicides led to a significant (P < 0.001) reduction in disease with an [Formula: see text] value of 1.198, indicating that, overall, fungicides were highly effective against cucurbit downy mildew. Fixed- and random-effects meta-analyses were then conducted to determine the effects of moderator variables on [Formula: see text]. The fixed-effects model resulted in narrower 95% confidence intervals and generally lower estimates of moderator subgroup mean effect size ([Formula: see text]) than the random-effects models. Fungicide efficacy was significantly (P < 0.001) greater in cucumber, with [Formula: see text] being 26.5% higher in cucumber than in other cucurbits. Year of study, number of sprays, and disease pressure in the control significantly (P < 0.001) affected [Formula: see text]. Fungicide efficacy was significantly lower during the 2004–05 season than prior to or after the 2004–05 season. Studies in which disease pressure was moderate had a significantly higher [Formula: see text] than studies with either low or high disease pressure. Fungicide efficacy was ≥22% in studies that received 5 to 6 sprays than in studies that received 1 to 4 or >6 sprays. Fungicide products led to a significant (P < 0.001) reduction in disease, although there was substantial between-study variability. The pyridinylmethyl-benzamide group of fungicides (fluopicolide) was the most effective, followed by the carbamate (propamocarb) and quinone inside inhibitors (cyazofamid) group of fungicides, while the carboxylic acid amide group (mandipropamid and dimethomorph) was the least effective. A combination of protectant and systemic fungicides resulted in a proportional increase in [Formula: see text] compared with sole application of either protectant or systemic fungicides.}, number={10}, journal={PHYTOPATHOLOGY}, publisher={Scientific Societies}, author={Ojiambo, P. S. and Paul, P. A. and Holmes, G. J.}, year={2010}, month={Oct}, pages={1066–1076} }
 @article{ojiambo_scherm_2010, title={Efficiency of Adaptive Cluster Sampling for Estimating Plant Disease Incidence}, volume={100}, ISSN={["0031-949X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77954317390&partnerID=MN8TOARS}, DOI={10.1094/phyto-100-7-0663}, abstractNote={ Conventional sampling designs such as simple random sampling (SRS) tend to be inefficient when assessing rare and highly clustered populations because most of the time is spent evaluating empty quadrats, leading to high error variances and high cost. In previous studies with rare plant and animal populations, adaptive cluster sampling, where sampling occurs preferentially in the neighborhood of quadrats in which the species of interest is detected during the sampling bout, has been shown to estimate population parameters with greater precision at an effort comparable to SRS. Here, we use computer simulations to evaluate the efficiency of adaptive cluster sampling for estimating low levels of disease incidence (0.1, 0.5, 1.0, and 5.0%) at various levels of aggregation of infected plants having variance-to-mean ratios (V/M) of ≈1, 3, 5, and 10. For each simulation, an initial sample size of 50, 100, and 150 quadrats was evaluated, and the condition to adapt neighborhood sampling (CA), i.e., the minimum number of infected plants per quadrat that triggers a switch from random sampling to sampling in neighboring quadrats, was varied from 1 to 4 (corresponding to 7.7 to 30.8% incidence of infected plants per quadrat). The simulations showed that cluster sampling was consistently more precise than SRS at a field-level disease incidence of 0.1 and 0.5%, especially when diseased plants were highly aggregated (V/M = 5 or 10) and when the most liberal condition to adapt (CA = 1) was used. One drawback of adaptive cluster sampling is that the final sample size is unknown at the beginning of the sampling bout because it depends on how often neighborhood sampling is triggered. In our simulations, the final sample size was close to the initial sample size for disease incidence up to 1.0%, especially when a more conservative condition to adapt (CA > 1) was used. For these conditions, the effect of disease aggregation was minor. In summary, both precision and the sample size required with adaptive cluster sampling responded similarly to disease incidence and aggregation, i.e., both were most favorable at the lowest disease incidence with the highest levels of clustering. However, whereas relative precision was optimized with the most liberal condition to adapt, the ratio of final to initial sample size was best for more conservative CA values, indicating a tradeoff. In our simulations, precision and final sample size were both simultaneously favorable for disease incidence of up to 1.0%, but only when infected plants were most aggregated (V/M = 10). }, number={7}, journal={PHYTOPATHOLOGY}, publisher={Scientific Societies}, author={Ojiambo, P. S. and Scherm, H.}, year={2010}, month={Jul}, pages={663–670} }
 @article{arauz_neufeld_lloyd_ojiambo_2010, title={Quantitative Models for Germination and Infection of Pseudoperonospora cubensis in Response to Temperature and Duration of Leaf Wetness}, volume={100}, ISSN={["1943-7684"]}, url={http://europepmc.org/abstract/AGR/IND44416978}, DOI={10.1094/phyto-100-9-0959}, abstractNote={ The influence of temperature and leaf wetness duration on germination of sporangia and infection of cantaloupe leaves by Pseudoperonospora cubensis was examined in three independent controlled-environment experiments by inoculating plants with a spore suspension and exposing them to a range of leaf wetness durations (2 to 24 h) at six fixed temperatures (5 to 30°C). Germination of sporangia was assessed at the end of each wetness period and infection was evaluated from assessments of disease severity 5 days after inoculation. Three response surface models based on modified forms of the Weibull function were evaluated for their ability to describe germination of sporangia and infection in response to temperature and leaf wetness duration. The models estimated 15.7 to 17.3 and 19.5 to 21.7°C as the optimum temperature (t) range for germination and infection, respectively, with little germination or infection at 5 or 30°C. For wetness periods of 4 to 8 h, a distinct optimum for infection was observed at t = 20°C but broader optimum curves resulted from wetness periods >8 h. Model 1 of the form f(w,t) = f(t) × (1 – exp{–[B × w]D}) resulted in smaller asymptotic standard errors and yielded higher correlations between observed and predicted germination and infection data than either model 2 of the form f(w,t) = A(1 – exp{– [f(t) × (w – C)]D}) or model 3 of the form f(w,t) = [1 – exp{–(B × w)2}]/cosh[(t – F)G/2]. Models 1 and 2 had nonsignificant lack-of-fit test statistics for both germination and infection data, whereas a lack-of-fit test was significant for model 3. The models accounted for ≈87% (model 3) to 98% (model 1) of the total variation in the germination and infection data. In the validation of the models using data generated with a different isolate of P. cubensis, slopes of the regression line between observed and predicted germination and infection data were not significantly different (P > 0.2487) and correlation coefficients between observed and predicted values were high (r2 > 0.81). Models 1 and 2 were used to construct risk threshold charts that can be used to estimate the potential risk for infection based on observed or forecasted temperature and leaf wetness duration. }, number={9}, journal={PHYTOPATHOLOGY}, author={Arauz, L. F. and Neufeld, K. N. and Lloyd, A. L. and Ojiambo, P. S.}, year={2010}, month={Sep}, pages={959–967} }
 @article{kanetis_holmes_ojiambo_2010, title={Survival of Pseudoperonospora cubensis sporangia exposed to solar radiation}, volume={59}, ISSN={["1365-3059"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77950937743&partnerID=MN8TOARS}, DOI={10.1111/j.1365-3059.2009.02211.x}, abstractNote={To determine the effects of solar radiation on the survival of sporangia of Pseudoperonospora cubensis, the cause of cucurbit downy mildew, detached sporangia of P. cubensis were either exposed to the sun or placed in the shade from 08·00 to 17·00 h on sunny and cloudy days at Raleigh, North Carolina, in experiments conducted weekly from 9 July to 8 October 2008. Sporangia were assessed for germination every hour following exposure and germination was normalized based on the germination percentage of unexposed sporangia from the same collection. Exposure for a 9‐h period, either in the sun or in the shade, significantly (P < 0·0001) reduced normalized germination of sporangia. Decrease in normalized germination was significantly (P < 0·0001) higher for sporangia exposed to the sun than sporangia in the shade, with the decrease being more rapid on sunny days than on cloudy days. On sunny days, solar radiation strongly reduced germination of sporangia (β = −3·51), whilst on cloudy days, reduction in germination of sporangia was minimal (β = −0·77). On sunny days, an exponential decay model, y = 95·2exp(−0·10x), provided a significant (R2 = 75·4%, n = 81, P < 0·0001) quantitative description of the effect of solar radiation dose on the percentage of normalized germination of sporangia. The effective cumulative solar radiation dose to inactivate 95% of the sporangia on sunny days was 29·5 MJ m−2. Disease severity data from inoculation experiments using sporangia exposed to varying amounts of cumulative solar radiation doses corroborated the results of the solar radiation experiments.}, number={2}, journal={PLANT PATHOLOGY}, publisher={Wiley}, author={Kanetis, L. and Holmes, G. J. and Ojiambo, P. S.}, year={2010}, month={Apr}, pages={313–323} }
 @article{zandjanakou-tachin_vroh-bi_ojiambo_tenkouano_gumedzoe_bandyopadhyay_2009, title={Identification and genetic diversity of Mycosphaerella species on banana and plantain in Nigeria}, volume={58}, ISSN={["1365-3059"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-65649153722&partnerID=MN8TOARS}, DOI={10.1111/j.1365-3059.2008.01988.x}, abstractNote={Ribosomal coding DNA was sequenced and compared in 95 isolates of Mycosphaerella spp. collected in Nigeria and single nucleotide polymorphism (SNP) was used to identify the species and to determine the genetic structure of the sampled geographical populations. Using reference GenBank accessions with intercontinental distributions as controls, and shared species‐specific SNPs in these control accessions, 84 (88·4%) isolates that grouped into 14 SNP haplotypes were identified as M. fijiensis, while 11 (11·6%) isolates represented by seven SNP haplotypes were characterized as M. eumusae. None of the isolates were either M. musicola or M. musae. The presence of M. fijiensis and M. eumusae in the collection was further confirmed using previously published species‐specific probes designed on actin and β‐tubulin gene sequences. A pairwise comparison of the population genetic distances revealed significant genetic differentiation between most populations (P < 0·001), with an average FST of 0·126, and a population structure corresponding to the four sampled geographical zones. The intraspecific dissimilarity of M. eumusae was 4·6%, compared with 2% for M. fijiensis. Compared to all the GenBank reference accessions, three sequence variations were unique to some Nigerian M. fijiensis haplotypes. Twenty‐one sequence haplotypes were identified, geographically mapped and registered in GenBank. The results indicate that M. musicola has been replaced by more frequently occurring M. fijiensis and M. eumusae, against which disease management and resistance breeding efforts should be directed in Nigeria.}, number={3}, journal={PLANT PATHOLOGY}, author={Zandjanakou-Tachin, M. and Vroh-Bi, I. and Ojiambo, P. S. and Tenkouano, A. and Gumedzoe, Y. M. and Bandyopadhyay, R.}, year={2009}, month={Jun}, pages={536–546} }
 @article{twizeyimana_ojiambo_sonder_ikotun_hartman_bandyopadhyay_2009, title={Pathogenic Variation of Phakopsora pachyrhizi Infecting Soybean in Nigeria}, volume={99}, ISSN={["1943-7684"]}, url={http://europepmc.org/abstract/AGR/IND44197711}, DOI={10.1094/PHYTO-99-4-0353}, abstractNote={Soybean rust, caused by Phakopsora pachyrhizi, is an important disease in Nigeria and many other soybean-producing countries worldwide. To determine the geographical distribution of soybean rust in Nigeria, soybean fields were surveyed in the Derived Savanna (DS), Northern Guinea Savanna (NGS), and Southern Guinea Savanna (SGS) agroecological zones in Nigeria between 2004 and 2006. Disease severity in each zone was determined and analyzed using geostatistics. Prevalence of infected fields and disease severity in surveyed fields were significantly (P < 0.05) different between geographical zones with both variables being higher in the DS zone than in either NGS or SGS zones. Geostatistical analysis indicated that the spatial influence of disease severity at one location on severity at other locations was between 75 and 120 km. An exponential model best described the relationship between semivariance and lag distance when rust severity was high. Spatial interpolation of rust severity showed that locations in the DS zone were more conducive for the rust epidemic compared to areas in the NGS zone. In the 2005 survey, 116 purified isolates were established in culture on detached soybean leaves. To establish the nature of pathogenic variation in P. pachyrhizi, a set of four soybean accessions with Rpp1, Rpp2, Rpp3, and Rpp4resistance genes, two highly resistant and two highly susceptible genotypes were inoculated with single uredinial isolates. Principal component analysis on the number of uredinia per square centimeter of leaf tissue for 116 isolates indicated that an adequate summary of pathogenic variation was obtained using only four genotypes. Of these four, PI 459025B (with Rpp4gene) and TG× 1485-1D had the lowest and highest number of uredinia per square centimeter, respectively. Based on cluster analysis of the number of uredinia per square centimeter, seven pathotype clusters were determined. Isolates in cluster III were the most virulent, while those in cluster IV were the least virulent. Shannon's index (H) revealed a more diverse pathogen population in the DS zone (H = 1.21) compared to the rust population in SGS and NGS with H values of 1.08 and 0.91, respectively. This work will be useful in breeding and management of soybean rust by facilitating identification of resistant genotypes and targeting cultivars with specific resistance to match prevailing P. pachyrhizi pathotypes in a given geographical zone.}, number={4}, journal={PHYTOPATHOLOGY}, author={Twizeyimana, M. and Ojiambo, P. S. and Sonder, K. and Ikotun, T. and Hartman, G. L. and Bandyopadhyay, R.}, year={2009}, month={Apr}, pages={353–361} }
 @article{almany_de arruda_arthofer_atallah_beissinger_berumen_bogdanowicz_brown_bruford_burdine_et al._2009, title={Permanent Genetic Resources added to Molecular Ecology Resources Database 1 May 2009-31 July 2009}, volume={9}, ISSN={["1755-098X"]}, url={http://europepmc.org/abstract/med/21564933}, DOI={10.1111/j.1755-0998.2009.02759.x}, abstractNote={Abstract}, number={6}, journal={MOLECULAR ECOLOGY RESOURCES}, author={Almany, Glenn R. and De Arruda, Mauricio P. and Arthofer, Wolfgang and Atallah, Z. K. and Beissinger, Steven R. and Berumen, Michael L. and Bogdanowicz, S. M. and Brown, S. D. and Bruford, Michael W. and Burdine, C. and et al.}, year={2009}, month={Nov}, pages={1460–1466} }
 @article{almany_de arruda_arthofer_atallah_beissinger_berumen_bogdanowicz_brown_bruford_burdine_et al._2009, title={Permanent genetic resources added to molecular ecology resources database 1 May 2009-31 July 2009}, volume={9}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70349920666&partnerID=MN8TOARS}, number={6}, journal={Molecular Ecology Resources}, author={Almany, G.R. and De Arruda, M.P. and Arthofer, W. and Atallah, Z.K. and Beissinger, S.R. and Berumen, M.L. and Bogdanowicz, S.M. and Brown, S.D. and Bruford, M.W. and Burdine, C. and et al.}, year={2009}, pages={1460–1466} }
 @misc{olanya_ojiambo_nyankanga_honeycutt_kirk_2009, title={Recent developments in managing tuber blight of potato (Solanum tuberosum) caused by Phytophthora infestans}, volume={31}, ISSN={["1715-2992"]}, DOI={10.1080/07060660909507602}, abstractNote={Tuber blight, caused by Phytophthora infestans, occurs wherever potato (Solanum tuberosum) is cultivated and accounts for significant losses under field and storage conditions. After decades of research in late blight, there has been substantial progress, but numerous challenges remain for the management of tuber blight. The tuber blight disease components comprising of infection pathways, tuber resistance, cultivar interactions, pathogen transmission, and survival on tubers are highlighted in this article. New fungicide chemistries, novel approaches, and cultural measures are presented along with the implications of pathogen diversity, cultivar differences, and edaphic factors on tuber blight incidence and control. With the application of molecular genetic tools in potato breeding to identify novel sources of resistance, effective control of the disease using host resistance is more likely. However, there are key elements of tuber infection and management that still need to be addressed. The quantitative relationships of inoculum load to tuber blight incidence and P. infestans interactions with edaphic factors are undetermined. Forecasting or development of predictive models for tuber blight incidence under field and storage environments will greatly improve disease management. Furthermore, quantification and characterization of the interactions between P. infestans and other storage pathogens (bacteria and fungi) or soil microbes, and the evaluation of biocontrol agents or alternative approaches for control of tuber blight may lead to effective management of this disease.}, number={3}, journal={CANADIAN JOURNAL OF PLANT PATHOLOGY}, author={Olanya, O. M. and Ojiambo, P. S. and Nyankanga, R. O. and Honeycutt, C. W. and Kirk, W. W.}, year={2009}, pages={280–289} }
 @article{olanya_ojiambo_nyankanga_honeycutt_kirk_2009, title={Recent developments in managing tuber blight of potato (Solanum tuberosum) caused by Phytophthora infestans}, volume={31}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77955871444&partnerID=MN8TOARS}, number={3}, journal={Canadian Journal of Plant Pathology}, author={Olanya, O.M. and Ojiambo, P.S. and Nyankanga, R.O. and Honeycutt, C.W. and Kirk, W.W.}, year={2009}, pages={280–289} }
 @article{vroh-bi_zandjanakou-tachin_m&apos_bah_tenkouano_ojiambo_bandyopadhyay_2009, title={Resistance of F1 segregating populations derived from crosses between wild banana accessions musa acuminata spp. burmannicoides 'calcutta 4' and m. balbisiana 'montpellier' to black leaf streak disease}, volume={828}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70350093172&partnerID=MN8TOARS}, journal={Acta Horticulturae}, author={Vroh-Bi, I. and Zandjanakou-Tachin, M. and M&apos and bah, W. and Tenkouano, A. and Ojiambo, P. and Bandyopadhyay, R.}, year={2009}, pages={353–358} }
 @article{nyankanga_olanya_wien_el-bedewy_karinga_ojiambo_2008, title={Development of tuber blight (Phytophthora infestans) on potato cultivars based on in vitro assays and field evaluations}, volume={43}, ISSN={["2327-9834"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-54249124087&partnerID=MN8TOARS}, DOI={10.21273/hortsci.43.5.1501}, abstractNote={Tuber blight may result from infection of wounded or unwounded potato tubers exposed to sporangia from foliar blight, soil, or blighted tubers. However, there are limited data on the prediction of tuber blight in field or storage environment based on in vitro assays. To assess this relationship, potato cultivars with foliar blight resistance (R-genes) and general resistance were evaluated for tuber blight incited by Phytophthora infestans (US-1) based on wound-induced and unwounded tuber inoculations. Surface lesion diameter, lesion depth, and frequency distribution of blighted tubers were assessed in in vitro assays and tuber blight incidence determined in field experiments. Significant differences (P < 0.05) in lesion diameter and depth were recorded among cultivars. Surface lesion diameter, depth, and index ranged from 5 to 40, 2 to 16.3, and 15 to 656 mm, respectively, in wound-inoculated tubers. In nonwounded tuber assays, the incidence of blighted tubers ranged from 0% to 8.7% in both years. Tuber blight infection of potato cultivars varied between years in field studies. Although tuber infection differed among cultivars, the frequency of blighted tubers had a normal statistical distribution irrespective of R-genes, implying that foliar resistance may have limited effect on tuber blight occurrence based on in vitro experiments. Prediction of tuber blight based on inoculation assays can be effectively used to estimate and manage blight development in storage environments.}, number={5}, journal={HORTSCIENCE}, author={Nyankanga, Richard O. and Olanya, Ocen Modesto and Wien, Hans C. and El-Bedewy, Ramzy and Karinga, John and OJiambo, Peter S.}, year={2008}, month={Aug}, pages={1501–1508} }
 @article{atehnkeng_ojiambo_donner_ikotun_sikora_cotty_bandyopadhyay_2008, title={Distribution and toxigenicity of Aspergillus species isolated from maize kernels from three agro-ecological zones in Nigeria}, volume={122}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-39149087032&partnerID=MN8TOARS}, number={1-2}, journal={International Journal of Food Microbiology}, author={Atehnkeng, J. and Ojiambo, P.S. and Donner, M. and Ikotun, T. and Sikora, R.A. and Cotty, P.J. and Bandyopadhyay, R.}, year={2008}, pages={74–84} }
 @article{atehnkeng_ojiambo_ikotun_sikora_cotty_bandyopadhyay_2008, title={Evaluation of atoxigenic isolates of Aspergillus flavus as potential biocontrol agents for aflatoxin in maize}, volume={25}, ISSN={["1944-0057"]}, url={http://europepmc.org/abstract/med/18608502}, DOI={10.1080/02652030802112635}, abstractNote={Aflatoxin contamination resulting from maize infection by Aspergillus flavus is both an economic and a public health concern. Therefore, strategies for controlling aflatoxin contamination in maize are being investigated. The abilities of eleven naturally occurring atoxigenic isolates in Nigeria to reduce aflatoxin contamination in maize were evaluated in grain competition experiments and in field studies during the 2005 and 2006 growing seasons. Treatments consisted of inoculation of either grains in vials or ears at mid-silking stage in field plots, with the toxigenic isolate (La3228) or atoxigenic isolate alone and co-inoculation of each atoxigenic isolate and La3328. Aflatoxin B1 + B2 concentrations were significantly (p < 0.05) lower in the co-inoculation treatments compared with the treatment in which the aflatoxin-producing isolate La3228 was inoculated alone. Relative levels of aflatoxin B1 + B2 reduction ranged from 70.1% to 99.9%. Among the atoxigenics, two isolates from Lafia, La3279 and La3303, were most effective at reducing aflatoxin B1 + B2 concentrations in both laboratory and field trials. These two isolates have potential value as agents for the biocontrol of aflatoxin contamination in maize. Because these isolates are endemic to West Africa, they are both more likely than introduced isolates to be well adapted to West African environments and to meet regulatory concerns over their use throughout that region.}, number={10}, journal={FOOD ADDITIVES AND CONTAMINANTS PART A-CHEMISTRY ANALYSIS CONTROL EXPOSURE & RISK ASSESSMENT}, author={Atehnkeng, J. and Ojiambo, P. S. and Ikotun, T. and Sikora, R. A. and Cotty, P. J. and Bandyopadhyay, R.}, year={2008}, pages={1264–1271} }
 @article{twizeyimana_ojiambo_ikotun_ladipo_hartman_bandyopadhyay_2008, title={Evaluation of soybean germplasm for resistance to soybean rust (Phakopsora pachyrhizi) in Nigeria}, volume={92}, ISSN={["1943-7692"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-46849092427&partnerID=MN8TOARS}, DOI={10.1094/PDIS-92-6-0947}, abstractNote={ Soybean rust, caused by Phakopsora pachyrhizi, is one of the most important constraints to soybean production worldwide. The absence of high levels of host resistance to the pathogen has necessitated the continued search and identification of sources of resistance. In one set of experiments, 178 soybean breeding lines from the International Institute of Tropical Agriculture were rated for rust severity in the field in 2002 and 2003 at Ile-Ife, Yandev, and Ibadan, Nigeria. Thirty-six lines with disease severity ≤3 (based on a 0-to-5 scale) were selected for a second round of evaluation in 2004 at Ibadan. In the third round of evaluation under inoculated field conditions, 11 breeding lines with disease severity ≤2 were further evaluated for rust resistance at Ibadan in 2005 and 2006. The breeding lines TGx 1835-10E, TGx 1895-50F, and TGx 1903-3F consistently had the lowest level of disease severity across years and locations. In another set of experiments, 101 accessions from the United States Department of Agriculture–Agricultural Research Service and National Agriculture Research Organization (Uganda) were evaluated in the first round in 2005 under inoculated conditions in the screenhouse; 12 accessions with disease severity ≤20% leaf area infected were selected for evaluation in the second round in 2005 and 2006 under inoculated field conditions at Ibadan. Highly significant differences (P < 0.0001) in disease severity were observed among the 101 accessions during this first round of rust evaluation. Significant (P < 0.0001) differences in rust severity and sporulation also were observed among the 12 selected accessions. Accessions PI 594538A, PI 417089A, and UG-5 had significantly (P < 0.05) lower disease severity than all other selected accessions in both years of evaluation, with rust severities ranging from 0.1 to 2.4%. These results indicate that some of the breeding lines (TGx 1835-10E, TGx 1895-50F, and TGx 1903-3F) and accessions (PI 594538A, PI 417089A, and UG-5) would be useful sources of soybean rust resistance genes for incorporation into high-yielding and adapted cultivars. }, number={6}, journal={PLANT DISEASE}, author={Twizeyimana, M. and Ojiambo, P. S. and Ikotun, T. and Ladipo, J. L. and Hartman, G. L. and Bandyopadhyay, R.}, year={2008}, month={Jun}, pages={947–952} }
 @article{afolabi_ojiambo_ekpo_menkir_bandyopadhyay_2008, title={Novel sources of resistance to Fusarium stalk rot of maize in tropical Africa}, volume={92}, ISSN={["1943-7692"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-44349103720&partnerID=MN8TOARS}, DOI={10.1094/PDIS-92-5-0772}, abstractNote={ Fusarium stalk rot is one of the most widespread and destructive diseases of maize, and deployment of resistant genotypes is one of the most effective strategies for controlling the disease. Fifty inbred lines and four checks from the breeding program of the International Institute of Tropical Agriculture were evaluated in field trials at Ikenne and Ibadan, Nigeria in 2003 and 2004 to identify new sources of resistance to stalk rot caused by Fusarium verticillioides. Evaluations were conducted under artificial inoculation and natural infection at Ibadan and Ikenne, respectively. Disease severity was recorded using a severity scale (SS) and direct estimation of stalk discoloration (SD). The two methods of disease assessment were compared and combined to classify genotypes into resistance groups using results from rank-sum analysis. In 2003, disease severity ranged from SS = 1 to 5 and SD = 1.3 to 33.8% at both locations. Both SS and SD were significantly (P < 0.01) higher in 2003 than in 2004 at the two locations. In both years, inbred lines significantly differed in SS (P < 0.02) and SD (P < 0.04) at Ibadan. Similarly, inbred lines significantly differed in SS (P < 0.04) and SD (P < 0.04) when genotypes were evaluated at Ikenne. Disease assessments based on SS and SD were significantly correlated (0.68 < r < 0.95, P < 0.01) in both years. Based on the results from rank-sum analysis, inbred lines were separated into highly resistant, resistant, moderately resistant, moderately susceptible, susceptible, and highly susceptible groups. At Ibadan, 6 (11.1%) and 8 (14.8%) were identified as highly resistant and resistant, respectively, whereas 11 (20.4%) were identified as resistant at Ikenne. Inbred lines 02C14609, 02C14643, 02C14654, and 02C14678 were consistently classified as either highly resistant or resistant to stalk rot across locations and years while the check genotypes were classified either as susceptible or moderately susceptible to stalk rot. These four inbred lines identified to have high levels of disease resistance may be used for breeding maize with resistance to Fusarium stalk rot. }, number={5}, journal={PLANT DISEASE}, publisher={Scientific Societies}, author={Afolabi, C. G. and Ojiambo, P. S. and Ekpo, E. J. A. and Menkir, A. and Bandyopadhyay, R.}, year={2008}, month={May}, pages={772–780} }
 @article{twizeyimana_ojiambo_ikotun_paul_hartman_bandyopadhyay_2007, title={Comparison of field, greenhouse, and detached-leaf evaluations of soybean germplasm for resistance to Phakopsora pachyrhizi}, volume={91}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34548220000&partnerID=MN8TOARS}, number={9}, journal={Plant Disease}, author={Twizeyimana, M. and Ojiambo, P.S. and Ikotun, T. and Paul, C. and Hartman, G.L. and Bandyopadhyay, R.}, year={2007}, pages={1161–1169} }
 @article{atehnkeng_ojiambo_donner_ikotun_sikora_cotty_bandyopadhyay_2007, title={Distribution and toxigenicity of Aspergillus species isolated from maize kernels from three agro-ecological zones in Nigeria.}, volume={2}, url={http://europepmc.org/abstract/med/18180068}, DOI={10.1016/j.ijfoodmicro.2007.11.062}, abstractNote={Maize samples were collected during a survey in three agro-ecological zones in Nigeria to determine the distribution and aflatoxin-producing potential of members of Aspergillus section Flavi. The three agro-ecological zones were, Derived Savannah (DS) and Southern Guinea Savannah (SGS) in the humid south and North Guinea Savannah (NGS) in the drier north. Across agro-ecological zones, Aspergillus was the most predominant fungal genera identified followed by Fusarium with mean incidences of 70 and 24%, respectively. Among Aspergillus, A. flavus was the most predominant and L-strains constituted >90% of the species identified, while the frequency of the unnamed taxon S(BG) was <3%. The incidence of atoxigenic strains of A. flavus was higher in all the districts surveyed except in the Ogbomosho and Mokwa districts in DS and SGS zones, respectively, where frequency of toxigenic strains were significantly (P<0.05) higher than that of atoxigenic strains. The highest and lowest incidence of aflatoxin positive samples was recorded in the SGS (72%) and NGS (20%), respectively. Aflatoxin contamination in grain also followed a similar trend and the highest mean levels of B-aflatoxins were detected in maize samples obtained from Bida (612 ng g(-1)) and Mokwa (169 ng g(-1)) districts, respectively, in the SGS. Similarly, the highest concentrations of G-aflatoxins were detected in samples from Akwanga district in the SGS with a mean of 193 and 60 ng g(-1), respectively. When agro-ecological zones were compared, B-aflatoxins were significantly (P<0.05) higher in SGS than in NGS, and intermediate in maize samples from the DS agro-ecological zone.}, journal={International journal of food microbiology}, author={Atehnkeng, J and Ojiambo, PS and Donner, M and Ikotun, T and Sikora, RA and Cotty, PJ and Bandyopadhyay, R}, year={2007}, month={Dec} }
 @article{afolabi_ojiambo_ekpo_menkir_bandyopadhyay_2007, title={Erratum: Evaluation of maize inbred lines for resistance to fusarium ear rot and fumonisin accumulation in grain in tropical Africa (Plant Disease (2007) 91 (3) (279-286))}, volume={91}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34247499048&partnerID=MN8TOARS}, number={5}, journal={Plant Disease}, author={Afolabi, C.G. and Ojiambo, P.S. and Ekpo, E.J.A. and Menkir, A. and Bandyopadhyay, R.}, year={2007} }
 @article{afolabi_ojiambo_ekpo_menkir_bandyopadhyay_2007, title={Evaluation of Maize Inbred Lines for Resistance to Fusarium Ear Rot and Fumonisin Accumulation in Grain in Tropical Africa}, volume={3}, url={http://europepmc.org/abstract/AGR/IND43898910}, DOI={10.1094/pdis-91-3-0279}, abstractNote={ Fusarium ear rot and fumonisin contamination is a major problem facing maize growers worldwide, and host resistance is the most effective strategy to control the disease, but resistant genotypes have not been identified. In 2003, a total of 103 maize inbred lines were evaluated for Fusarium ear rot caused by Fusarium verticillioides in field trials in Ikenne and Ibadan, Nigeria. Disease was initiated from natural infection in the Ikenne trial and from artificial inoculation in the Ibadan trial. Ear rot severity ranged from 1.0 to 6.0 in both locations in 2003. Fifty-two inbred lines with disease severity ≤3 (i.e., ≤ 10% visible symptoms on ears) were selected and reevaluated in 2004 for ear rot resistance, incidence of discolored kernels, and fumonisin contamination in grain. At both locations, ear rot severity on the selected lines was significantly (P < 0.0020) higher in 2004 than in 2003. The effects of selected inbred lines on disease severity were highly significant at Ikenne (P = 0.0072) and Ibadan (P < 0.0001) in 2004. Inbred lines did not affect incidence of discolored kernels at both locations and across years except at Ikenne (P = 0.0002) in 2004. Similarly, significant effects of inbred lines on fumonisin concentration were observed only at Ikenne (P = 0.0201) in 2004. However, inbred lines 02C14585, 02C14593, 02C14603, 02C14606, 02C14624, and 02C14683 had consistently low disease severity across years and locations. Fumonisin concentration was significantly correlated with ear rot only at Ikenne (R = 0.42, P < 0.0001). Correlation between fumonisin concentration and incidence of discolored kernels was also significant at Ikenne (R = 0.39, P < 0.0001) and Ibadan (R = 0.35, P = 0.0007). At both locations, no significant inbred × year interaction was observed for fumonisin concentration. Five inbred lines, namely 02C14585, 02C14603, 02C14606, 02C14624, and 02C14683, consistently had the lowest fumonisin concentration in both trials. Two of these inbred lines, 02C14624 and 02C14585, had fumonisin levels <5.0 μg/g across years in trials where disease was initiated from both natural infection and artificial inoculation. These lines that had consistently low disease severity are useful for breeding programs to develop fumonisin resistant lines. }, number={3}, journal={Plant disease}, author={Afolabi, CG and Ojiambo, PS and Ekpo, EJA and Menkir, A and Bandyopadhyay, R}, year={2007}, month={Mar}, pages={279–286} }
 @article{ojiambo_bandyopadhyay_twizeyimana_lema_frederick_pedley_stone_hartman_2007, title={First Report of Rust Caused by Phakopsora pachyrhizi on Soybean in Democratic Republic of Congo.}, volume={9}, url={http://europepmc.org/abstract/med/30780681}, DOI={10.1094/pdis-91-9-1204c}, abstractNote={ Nigeria (1) and Uganda (3) are the closest countries to the Democratic Republic of Congo (DRC) where soybean rust caused by Phakopsora pachyrhizi has been reported. In February 2007, during a disease survey in DRC, soybean (Glycine max) leaves with rust symptoms (tan, angular lesions with erumpent sori exuding urediniospores) were observed in 10 fields in the following areas in Bas Congo Province: Bangu, Kimpese, Kolo-Fuma, Lukala, Mbanza-Ngungu, Mpalukide, Mvuazi, and Ntemo. Rust incidence in these fields ranged from 85 to 100%, while severity ranged between 3 and 35% of the leaf area on infected plants. Urediniospores were hyaline, minutely echinulate, and 23 to 31 × 16 to 20 μm. Within a week of collection, infected leaf samples were sent to the USDA-ARS Foreign Disease-Weed Science Research Unit (FDWSRU) for pathogen identification. DNA was extracted from sections of leaves containing sori with the Qiagen DNeasy Plant Mini kit (Valencia, CA), and all 10 field samples amplified in a real-time fluorescent PCR with the P. pachyrhizi-specific primers Ppm1 and Ppa2 (2). Infected leaves of cultivar Vuangi collected from one field each in the INERA Research Station, Kimpese-Crawford, and Kimpese-Ceco were separately washed in sterile water to collect urediniospores that were used to separately inoculate three detached leaves of susceptible cultivar TGx 1485-1D (4). Lesions on inoculated leaves developed 5 days after inoculation (DAI), and pustules (110 to 130 μm) formed 7 DAI and erupted 2 days later exuding columns of urediniospores similar in size to the initially collected isolates. Inoculation of another set of detached leaves with a spore suspension (1 × 106 spores per ml) from the first set of detached leaves resulted in typical rust symptoms. Seedlings of cultivar Williams also showed typical rust symptoms when inoculated separately with urediniospores collected from nine fields (i.e., all except Kimpese-Ceco, which was infective in the detached leaf assay). Inoculation and incubation were carried out at the FDWSRU Plant Pathogen Containment Facility at Fort Detrick as described earlier (2). The PCR assay, morphological characters of the isolates, and pathogenicity tests demonstrate that P. pachyrhizi occurs in DRC. To our knowledge, this is the first report of P. pachyrhizi infecting soybean in DRC. }, journal={Plant disease}, author={Ojiambo, PS and Bandyopadhyay, R and Twizeyimana, M and Lema, A and Frederick, RD and Pedley, KF and Stone, CL and Hartman, GL}, year={2007}, month={Sep} }
 @article{bandyopadhyay_ojiambo_twizeyimana_asafo-adjei_frederick_pedley_stone_hartman_2007, title={First Report of Soybean Rust Caused by Phakopsora pachyrhizi in Ghana.}, volume={8}, url={http://europepmc.org/abstract/med/30780460}, DOI={10.1094/pdis-91-8-1057b}, abstractNote={ Nigeria is the only country in West Africa where soybean rust, caused by Phakopsora pachyrhizi, has been officially reported (1). During a disease survey in Ghana during October 2006, soybean (Glycine max) leaves with rust symptoms (tan, angular lesions with erumpent sori exuding urediniospores) were observed in 11 fields in the following districts: Kassena Nankana in the Upper East Region; East Gonja, Central Gonja, and Tolon-Kumbungu in the Northern Region; and Ejisu-Juabeng in the Ashanti Region. Disease incidence in these fields ranged from 50 to 100% and disease severity ranged between 3 and 40% of the leaf area on infected plants. Urediniospores were hyaline, minutely echinulate, and 23 to 31 × 14 to 18 μm. Within a week of collection, leaf samples were sent to the USDA-ARS Foreign Disease-Weed Science Research Unit for verification of pathogen identity. DNA was extracted from leaf pieces containing sori with the Qiagen DNeasy Plant Mini kit (Valencia, CA), and all 11 field samples amplified in a real-time fluorescent PCR with the P. pachyrhizi-specific primers Ppm1 and Ppa2 (2). Sequence alignment of the internal transcribed spacer (ITS) region 2 further confirmed the identification as P. pachyrhizi (2). Infected leaves from three fields were separately washed in sterile water to collect urediniospores that were used to separately inoculate three detached leaves (for each isolate) of susceptible cultivar TGx 1485-1D (3). The abaxial surface of detached leaves was sprayed with 400 μl of spore suspension (1 × 106 spores per ml). A single leaf piece was placed in a 9-cm-diameter petri dish with adaxial side appressed on 1% technical agar amended with 10 μg/ml of kinetin. Lactic acid (1.5 ml/liter) and benomyl (12.5 mg/liter) were added to the agar medium to inhibit growth of saprophytic fungi and bacteria. Petri dishes were incubated at 20°C with a 12-h light/12-h dark cycle. Lesions on inoculated leaves developed 5 to 6 days after inoculation (DAI), and pustules (105 to 120 μm) formed 7 to 8 DAI and erupted 3 days later exuding columns of urediniospores similar in size to the initially collected isolates. Inoculating another set of detached leaves with a spore suspension (1 × 106 spores per ml) from the first set of detached leaves resulted in typical rust symptoms. The PCR assay, alignment of ITS region 2, morphological characters of the isolates, and pathogenicity tests demonstrate that P. pachyrhizi occurs in Ghana. To our knowledge, this is the first report of P. pachyrhizi in Ghana. }, journal={Plant disease}, author={Bandyopadhyay, R and Ojiambo, PS and Twizeyimana, M and Asafo-Adjei, B and Frederick, RD and Pedley, KF and Stone, CL and Hartman, GL}, year={2007}, month={Aug} }
 @article{twizeyimana_ojiambo_tenkouano_ikotun_bandyopadhyay_2007, title={Rapid Screening of Musa Species for Resistance to Black Leaf Streak Using In Vitro Plantlets in Tubes and Detached Leaves}, volume={3}, url={http://europepmc.org/abstract/AGR/IND43898921}, DOI={10.1094/pdis-91-3-0308}, abstractNote={ This study investigated the utility of inoculation of in vitro plantlets in tubes and detached leaves as reliable and rapid assays for screening Musa genotypes against Mycosphaerella fijiensis, the causal agent of black leaf streak. In the first part of the study, three types of inocula were evaluated to determine suitability for in vitro inoculation. Inoculation of in vitro plantlets with mycelial fragments resulted in significantly (P < 0.05) higher levels of disease severity and faster rates of disease progress compared with inoculations using conidial suspensions. In the detached leaf assay, amending agar medium with plant hormones significantly (P < 0.0001) aided retention of green leaf color. Leaf pieces on medium containing gibberellic acid at 5 mg/liter had about 5% chlorosis at 52 days after plating. When in vitro plantlets in tubes and detached leaves of 10 Musa genotypes with different levels of disease resistance were inoculated with M. fijiensis, there were significant (P < 0.05) differences among genotypes in leaf area infected, incubation time, and symptom evolution time. For incubation time and leaf area infected, cultivars responded depending on their level of disease resistance, with resistant genotypes Calcutta-4 and PITA-17 having significantly (P = 0.001) longer incubation times and lower infected leaf areas compared with the susceptible cultivar Agbagba and moderately resistant cultivar FHIA-23. A similar pattern in cultivar response was observed for symptom evolution time. Leaf area infected was not significantly (P = 0.2817 for two-tailed t test) different when assessed using the two assays, and infected leaf areas in both assays were strongly correlated (r = 0.88, n = 48, P < 0.0001). Although incubation times were significantly (P = 0.0062 for two-tailed t test) different between the two assays, values from the two assays were strongly correlated (r = 0.69, n = 48, P < 0.0001). These results show that these two assays are rapid and space-effective, and can reliably be used for screening Musa genotypes for resistance to black leaf streak. }, number={3}, journal={Plant disease}, author={Twizeyimana, M and Ojiambo, PS and Tenkouano, A and Ikotun, T and Bandyopadhyay, R}, year={2007}, month={Mar}, pages={308–314} }
 @article{nyankanga_wien_olanya_ojiambo_2007, title={Relationship between late blight [Phytophthora infestans] of potato on tuber and foliage, as affected by the disease severity on foliage, cultivar resistance, and atmospheric and soil variables}, volume={29}, ISSN={0706-0661 1715-2992}, url={http://dx.doi.org/10.1080/07060660709507483}, DOI={10.1080/07060660709507483}, abstractNote={Potato tuber blight, caused by Phytophthora infestans, is an important component of the late blight pathosystem. Although the dynamics of tuber blight on potato (Solanum tuberosum) cultivars have been evaluated, the effects of climatic and soil variables on tuber blight incidence have not been adequately quantified. Field experiments were conducted at two distinct environments: in New York (1998–1999) using the A2 mating type (US-8 clonal lineage), and in Kenya (2000–2001) using the A1 mating type (US-1 clonal lineage). Fungicide applications were scheduled to vary the amount of blight infection. Foliar and tuber blight development, climatic variables, and soil variables were quantified during the cropping seasons. Fungicide application did not have a significant effect on the incidence of tuber blight at both locations. Soil temperature, precipitation, tuber depth, and cultivar resistance were significantly correlated with incidence of tuber blight at both locations. At Freeville, New York, precipitation when soil temperature was 16–18 °C had the highest correlation (r 2 = 0.632) with tuber blight, whereas soil moisture had the highest correlation (r 2 = 0.577) with tuber blight in Kenya. Path coefficient analysis showed that total precipitation during the epidemics and days when soil temperature was 16–18 °C had the largest direct effect on tuber blight in New York and at the field sites in Kenya. Regression models using atmospheric variables, soil variables, and cultivar resistance had moderate predictive ability of tuber blight at New York (0.44 < R 2 < 0.61) but low prediction in Kenya (0.40 < R 2 < 0.46). Similarly, cultivar specific models using foliar blight, atmospheric variables, and soil variables resulted in significant predictions of tuber blight in New York (R 2 > 0.46), whereas few regression equations for Kenya resulted in significant prediction of tuber blight. These results suggest that cultivar resistance, soil variables, and atmospheric variables are the main determinants of foliar and tuber blight infection when inoculum is present. Key words: Phytophthora infestans, Solanum tuberosum, tuber blight, cultivars, environmental factors, resistance. La brûlure du tubercule de la pomme de terre, causée par le Phytophthora infestans, est une importante composante du pathosystème du mildiou. Quoique la dynamique de la brûlure du tubercule de la pomme de terre (Solanum tuberosum) sur les cultivars ait été étudiée, les effets des variables climat et sol sur l'incidence de la brûlure du tubercule n'ont pas été suffisamment quantifiés. Des essais sur le terrain furent menés dans deux environnements distincts : dans l'état de New York (1998–1999) avec le type sexuel A2 (lignage clonal US-8) et au Kenya (2000–2001) avec le type sexuel A1 (lignage clonal US-1). Des applications de fongicides furent programmées de façon à faire varier la quantité de brûlure. Les variables développement de la brûlure des feuilles et du tubercule, climat et sol furent quantifiées pendant les saisons de végétation. L'application de fongicides n'a pas eu d'effets significatifs sur l'incidence de la brûlure du tubercule aux deux endroits. La température du sol, les précipitations, la profondeur des tubercules et la résistance des cultivars étaient significativement corrélées avec l'incidence de la brûlure du tubercule aux deux endroits. À Freeville, New York, les précipitations lorsque la température du sol était située entre 16 et 18 °C furent les plus fortement corrélées (r 2 = 0,632) avec la brûlure du tubercule, alors que l'humidité du sol fut la plus fortement corrélée (r 2 = 0,577) avec la brûlure du tubercule au Kenya. L'analyse des coefficients de direction montra que les précipitations totales pendant les épidémies et pendant les jours où les températures du sol étaient de 16 à 18 °C eurent le plus gros effet direct sur la brûlure du tubercule aux lieux des essais de l'état de New York et du Kenya. Des modèles de régression basés sur des variables atmosphériques et telluriques et sur la résistance variétale furent moyennement capables de prédire la brûlure du tubercule dans l'état de New York (0,44 < R 2 > 0,61), mais ne le furent que faiblement au Kenya (0,40 < R 2 > 0,46). De même, des modèles de régression spécifiques au cultivar et basés sur les variables brûlure des feuilles, atmosphère et sol ont généré des prédictions significatives de la brûlure du tubercule dans l'état de New York (R 2 > 0,46), alors qu'au Kenya, peu d'équations de régression ont donné des prédictions significatives de la brûlure du tubercule. Ces résultats suggèrent que les variables résistance variétale, sol et atmosphère sont les principaux déterminants de l'infection des feuilles et du tubercule lorsque de l'inoculum est présent. Mots-clés : Phytophthora infestans, Solanum tuberosum, brûlure du tubercule, cultivars, facteurs environnementaux, résistance.}, number={4}, journal={Canadian Journal of Plant Pathology}, publisher={Informa UK Limited}, author={Nyankanga, R. O. and Wien, H. C. and Olanya, O. M. and Ojiambo, P. S.}, year={2007}, month={Dec}, pages={372–387} }
 @article{ojiambo_scherm_brannen_2007, title={Temporal Dynamics of Septoria Leaf Spot of Blueberry and its Relationship to Defoliation and Yield}, volume={8}, DOI={10.1094/php-2007-0726-05-rs}, abstractNote={ In field trials on Premier rabbiteye blueberry in Georgia, onset of Septoria leaf spot (caused by Septoria albopunctata) occurred between late April and mid-June. Average disease severity increased sigmoidally until late September, after which it declined due to the abscission of severely affected leaves. Disease severity was highest on early-emerging leaves and on those located on shoots closer to the ground. Pycnidiospore inoculum was present throughout the season, and leaves became infected by S. albopunctata season-long. Disease severity, defoliation, flower bud set, and next season's yield were interrelated; severely affected leaves abscised earlier in the fall than those with low disease severity, and shoots with severely diseased leaves and/or high levels of defoliation had reduced flower bud set. Furthermore, such shoots consistently had low yields the following year. The results form the basis for identifying disease levels that can be tolerated during specific periods of crop development without negatively impacting flower bud set and yield. }, number={1}, journal={Plant Health Progress}, publisher={Scientific Societies}, author={Ojiambo, Peter S. and Scherm, Harald and Brannen, Phillip M.}, year={2007}, month={Jan}, pages={68} }
 @article{ojiambo_2007, title={Temporal Dynamics of Septoria Leaf Spot of Blueberry and its Relationship to Defoliation and Yield}, publisher={Plant Health Progress}, author={Ojiambo, P.}, year={2007} }
 @article{ojiambo_scherm_2006, title={Biological and application-oriented factors influencing plant disease suppression by biological control: a meta-analytical review.}, volume={11}, url={http://europepmc.org/abstract/med/18943952}, DOI={10.1094/phyto-96-1168}, abstractNote={ Studies to evaluate the effectiveness of biological control in suppressing plant disease often report inconsistent results, highlighting the need to identify general factors that influence the success or failure of biological control in plant pathology. We conducted a quantitative synthesis of previously published research by applying meta-analysis to determine the overall effectiveness of biocontrol in relation to biological and application-oriented factors. For each of 149 entries (antagonist-disease combinations) from 53 reports published in Biological & Cultural Tests between 2000 and 2005, an effect size was calculated as the difference in disease intensity expressed in standard deviation units between the biocontrol treatment and its corresponding untreated control. Effect sizes ranged from -1.15 (i.e., disease strongly enhanced by application of the biocontrol agent) to 4.83 (strong disease suppression by the antagonist) with an overall weighted mean of 0.62, indicating moderate effectiveness on average. There were no significant (P >0.05) differences in effect sizes between entries from studies carried out in the greenhouse versus the field, between those involving soilborne versus aerial diseases, or among those carried out in conditions of low, medium, or high disease pressure (expressed relative to the disease intensity in the untreated control). However, effect sizes were greater on annual than on perennial crops, regardless of whether the analysis was carried out for all entries (P = 0.0268) or for those involving only soilborne diseases (P = 0.0343). Effect sizes were not significantly different for entries utilizing fungal versus bacterial biocontrol agents or for those targeting fungal versus bacterial pathogens. However, entries that used r-selected biological control agents (i.e., those having short generation times and producing large numbers of short-lived offspring) were more effective than those that applied antagonists that were not r-selected (P = 0.0312). Interestingly, effect sizes for entries that used Bacillus spp. as biological control agents were lower than for those that applied other antagonists (P = 0.0046 for all entries and P = 0.0114 for soilborne diseases). When only aerial diseases were considered, mean effect size was greater for entries that received one or two sprays than for those that received more than eight sprays of the biocontrol agent (P = 0.0002). This counterintuitive result may indicate that investigators often attempt unsuccessfully to compensate for anticipated poor performance in antagonist-disease combinations by making more applications. }, number={11}, journal={Phytopathology}, author={Ojiambo, PS and Scherm, H}, year={2006}, month={Nov}, pages={1168–1174} }
 @article{olanya_ojiambo_nyankanga_2006, title={Dynamics of development of late blight [Phytophthora infestans] in potato, and comparative resistance of cultivars in the highland tropics.}, volume={1}, url={http://europepmc.org/abstract/AGR/IND43834169}, DOI={10.1080/07060660609507274}, abstractNote={Host resistance is an important component for the management of late blight [Phytophthora infestans] on potato, in the highland tropics, where effective fungicide use is limited because of the cost of application. Potato cultivars with major resistance genes to late blight (population A) and minor or quantitative resistance genes to the disease (population B) were evaluated in field studies at two locations in Kenya during 2000, 2001, and 2002 cropping seasons. Disease severity, area under disease progress curve (AUDPC), infection rates, tuber blight, and tuber yields were assessed to determine the effectiveness of cultivar resistance to potato late blight. Significant differences (P < 0.05) in AUDPC were detected among cultivars. Progress of late blight on potato cultivars was best described by the nonlinear form of the logistic model. Infection rates ranged from 0.0047 to 0.3105 logits per day and were generally higher on susceptible than resistant cultivars. Rates of disease progress were highest on the susceptible control 'Kerr's Pink' (0.3015 logits per day) and lowest on ‘Rutuku’ (0.0047 logits per day), a cultivar derived from population A. The rates of disease progress on population B cultivars were significantly (P < 0.001) higher than those on population A in seasons of severe late-blight epidemics. Significant (P < 0.05) differences in tuber blight development were also detected among the cultivars. Significant negative correlations were observed between AUDPC and tuber yield for cultivars of population A, but no correlations were observed for cultivars of population B. This suggests that cultivars of population B are less influenced in their yield by late blight than those of population A and may be more suited for use where late blight is a recurring problem.}, journal={Canadian journal of plant pathology. Revue Canadienne de phytopathologie}, author={Olanya, OM and Ojiambo, PS and Nyankanga, RO}, year={2006}, month={Jan} }
 @article{ojiambo_scherm_2006, title={Optimum Sample Size for Determining Disease Severity and Defoliation Associated with Septoria Leaf Spot of Blueberry}, volume={90}, url={http://europepmc.org/abstract/med/30781103}, DOI={10.1094/pd-90-1209}, abstractNote={ In a 3-year field study, Premier rabbiteye blueberry plants were sampled at three hierarchical levels (leaf, shoot, and bush) to assess severity of Septoria leaf spot (caused by Septoria al-bopunctata) and incidence of defoliation. A positive linear relationship (R 2 = 0.977, P < 0.0001, n = 2127) was observed between the number of spots per leaf and percent necrotic leaf area, both assessed on individual leaves in mid- to late October. For data summarized at the shoot level, percent defoliation increased nonlinearly (R 2 = 0.729, P < 0.0001, n = 224) as disease severity increased, with a rapid rise to an upper limit showing little change in defoliation above 60 spots per leaf. Variance components were calculated for disease severity to partition total variation into variation among leaves per shoot, shoots per bush, and bushes within the field. In all cases, leaves per shoot and shoots per bush accounted for >90% of the total variation. Based on the variance components and linear cost functions (which considered the time required to assess each leaf and select new shoots and bushes for assessment), the optimum sample size for assessing disease severity as number of spots per leaf (with an allowable variation of 20% around the mean) was 75 leaves, one each selected from three shoots per bush on 25 bushes (total time required for assessment: 36.1 min). For disease severity expressed as percent necrotic leaf area, the corresponding values were 144 leaves, two each sampled from three shoots per bush on 24 bushes (total time required: 21.7 min). Thus, given the strong correlation between the two disease variables demonstrated in this study, visual assessment of percent necrotic area was the more efficient method. With an allowable variation of 10% around the mean, a sample of 27 shoots from nine bushes was the optimum sample size for assessing defoliation across the 3 years. }, number={9}, journal={Plant Disease}, publisher={Scientific Societies}, author={Ojiambo, P. S. and Scherm, H.}, year={2006}, month={Sep}, pages={1209–1213} }
 @article{ojiambo_scherm_brannen_2006, title={Septoria Leaf Spot Reduces Flower Bud Set and Yield Potential of Rabbiteye and Southern Highbush Blueberries}, volume={90}, url={http://europepmc.org/abstract/AGR/IND43786129}, DOI={10.1094/pd-90-0051}, abstractNote={In field trials on Premier rabbiteye blueberry, individual shoots were selected and tagged in the fall of 2001, 2002, and 2003 to quantify the effects of Septoria leaf spot severity and disease-induced premature defoliation on flower bud set and return yield. Experiments were carried outsimilarly on Bluecrisp southern highbush blueberry using shoots tagged after fruit harvest in the summer of 2002 and 2003. Leaves on the distal 20-cm segments of these shoots were monitored for disease severity (number of spots per leaf) through the remainder of the growing season; at the same time, defoliation (expressed as the proportion of nodes with missing leaves) was recorded for each of the shoot segments. Flower bud set was assessed subsequently in winter or early spring, and berries were harvested as they matured the following summer to determine return yield. For both cultivars, higher flower bud numbers were more likely to occur on shoots with lower disease levels the previous fall (P ≤ 0.0462 based on a Kolmogorov-Smirnov test). The data further showed that flower bud set potential (i.e., the maximum number of buds on shoots within a given disease severity range) decreased linearly as disease severity increased (r2≥ 0.926, P ≤ 0.0005). Based on the slope of this relationship, flower bud set potential decreased by one bud per shoot as disease severity the previous fall increased by 18 and 12 spots per leaf for Premier and Bluecrisp, respectively. Relationships between yield and disease variables were similar to those of flower bud numbers and disease, except that the decrease in yield potential (i.e., the maximum fruit weight per shoot within a given disease severity range) was less gradual than for flower bud set potential. On Premier, yield potential dropped markedly and significantly as disease severity the previous fall exceeded about 50 to 60 spots per leaf on average (P < 0.0001 based on a Kruskal-Wallis test). Evidence for such a threshold effect was weaker on Bluecrisp, presumably because of the lower number of data points for this cultivar combined with lower yields due to poor pollination.}, number={1}, journal={Plant Disease}, publisher={Scientific Societies}, author={Ojiambo, P. S. and Scherm, H. and Brannen, P. M.}, year={2006}, month={Jan}, pages={51–57} }
 @article{scherm_ngugi_ojiambo_2006, title={Trends in Theoretical Plant Epidemiology}, volume={5}, url={http://europepmc.org/abstract/AGR/IND43819451}, DOI={10.1007/s10658-005-3682-6}, number={1}, journal={European journal of plant pathology.}, author={Scherm, H and Ngugi, HK and Ojiambo, PS}, year={2006}, month={May}, pages={61–73} }
 @article{ojiambo_scherm_2005, title={Survival Analysis of Time to Abscission of Blueberry Leaves Affected by Septoria Leaf Spot}, volume={95}, url={http://europepmc.org/abstract/AGR/IND43679599}, DOI={10.1094/phyto-95-0108}, abstractNote={In the southeastern United States, Septoria leaf spot, caused by Septoria albopunctata, can result in premature defoliation of blueberry plants during summer and fall, thereby reducing yield potential for the following year. The effects of disease severity and leaf attributes (leaf age and leaf location in the canopy) on the dynamics (timing and extent) of defoliation were quantified in field plots of Premier rabbiteye blueberry (Vaccinium ashei) in 2002 and 2003. In each year, 50 shoots were selected for assessment in early spring, and all leaves on these shoots (n = 410 and 542 in 2002 and 2003, respectively) were monitored individually for disease progress and time of abscission at 3- to 10-day intervals throughout the season. In both years, disease progress was characterized by an exponential increase in disease severity up to late September, followed by a decline toward the end of the assessment period in late November. Defoliation was sporadic up to late August, followed by more rapid and sustained levels of leaf loss. Abscission of severely infected leaves could explain the decline in disease severity toward the end of the season. Final disease severity (i.e., disease severity on the last assessment date before leaf drop) was highest for leaves that abscised early and lowest for leaves that had not abscised by the end of the assessment period. Survival analysis revealed that older leaves (located on the lower halves of shoots) and leaves with high levels of disease (≥5 spots/leaf at the time of fruit harvest in mid-June) abscised significantly (P < 0.0001) earlier than younger leaves and leaves with lower disease severity. Relative to their respective reference groups, mean times to abscission were ≈2 weeks shorter for the older leaf group and ≈3 weeks shorter in the leaf group afflicted by high disease severity. When an accelerated failure time model was fitted to the data, the resulting parameter estimates indicated that each additional leaf spot present at harvest accelerated time to leaf abscission (expressed using late August as a starting point) by 1.9 and 4.5% in 2002 and 2003, respectively. Leaf location in upper or lower portions of the canopy had no significant effect on time to abscission (P > 0.05).}, number={1}, journal={Phytopathology}, publisher={Scientific Societies}, author={Ojiambo, P. S. and Scherm, H.}, year={2005}, month={Jan}, pages={108–113} }
 @article{ojiambo_scherm_2005, title={Temporal Progress of Septoria Leaf Spot on Rabbiteye Blueberry (Vaccinium ashei)}, volume={89}, url={http://europepmc.org/abstract/med/30791277}, DOI={10.1094/pd-89-1090}, abstractNote={ Septoria leaf spot, caused by Septoria albopunctata, is an important disease on blueberry in the southeastern United States, yet its epidemiology is largely unknown. Disease severity and dissemination of pycnidiospores were monitored from 2002 to 2004 in a planting of susceptible Premier rabbiteye blueberry to characterize the temporal progress of the disease and determine the effect of inoculum dynamics and selected leaf attributes on disease development. Disease onset was observed between late April and mid-June, followed by a rapid increase in disease severity until mid- to late September; thereafter, disease severity decreased until the end of the season due to abscission of severely infected leaves. A logistic model was fitted to disease severity data using nonlinear regression, and parameter estimates were used to compare the effects of leaf position on the shoot and shoot location in the canopy on disease progress. Based on this model, the highest absolute rate of disease increase and the highest upper asymptote of disease severity were predicted for leaves in intermediate positions on the shoot and for shoots in the lower canopy. Data collected with funnel spore samplers showed that splash-dispersed pycnidiospores of S. albopunctata were available throughout most of the period from April through late October. Final disease severity on individual leaves was more strongly correlated with cumulative spore numbers throughout the entire season (from leaf emergence to the end of the assessment period in November) than with cumulative spore numbers during shorter periods around the time of leaf emergence; this suggests that infection is not limited to young, expanding leaves, but rather that leaves at all developmental stages can become infected by S. albopunctata seasonlong. Disease incidence on leaves of potted trap plants exposed to natural inoculum in the field during rain events in 2003 and 2004 was >70.0% irrespective of leaf developmental stage at the time of exposure. Taken together, the results of this study indicate that inoculum of S. albopunctata is present throughout most of the growing season and that infection can occur season-long on leaves of any age, giving rise to a polycyclic epidemic. }, number={10}, journal={Plant Disease}, publisher={Scientific Societies}, author={Ojiambo, P. S. and Scherm, H.}, year={2005}, month={Oct}, pages={1090–1096} }
 @article{scherm_ojiambo_2004, title={Applications of Survival Analysis in Botanical Epidemiology}, volume={94}, url={http://europepmc.org/abstract/AGR/IND43645293}, DOI={10.1094/phyto.2004.94.9.1022}, abstractNote={ Data on the occurrence and timing of discrete events such as spore germination, disease onset, or propagule death are recorded commonly in epidemiological studies. When analyzing such “time-to-event” data, survival analysis is superior to conventional statistical techniques because it can accommodate censored observations, i.e., cases in which the event has not occurred by the end of the study. Central to survival analysis are two mathematical functions, the survivor function, which describes the probability that an individual will “survive” (i.e., that the event will not occur) until a given point in time, and the hazard function, which gives the instantaneous risk that the event will occur at that time, given that it has not occurred previously. These functions can be compared among two or more groups using chi-square-based test statistics. The effects of discrete or continuous covariates on survival times can be quantified with two types of models, the accelerated failure time model and the proportional hazards model. When applied to longitudinal data on the timing of defoliation of individual blueberry leaves in the field, analysis with the accelerated failure time model revealed a significantly (P < 0.0001) increased defoliation risk due to Septoria leaf spot, caused by Septoria albopunctata. Defoliation occurred earlier for lower leaves than for upper leaves, but this effect was confounded in part with increased disease severity on lower leaves. }, number={9}, journal={Phytopathology}, publisher={Scientific Societies}, author={Scherm, H. and Ojiambo, P. S.}, year={2004}, month={Sep}, pages={1022–1026} }
 @article{nyankanga_wien_olanya_ojiambo_2004, title={Farmers' cultural practices and management of potato late blight in Kenya highlands: Implications for development of integrated disease management}, volume={50}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-2542427836&partnerID=MN8TOARS}, number={2}, journal={International Journal of Pest Management}, author={Nyankanga, R.O. and Wien, H.C. and Olanya, O.M. and Ojiambo, P.S.}, year={2004}, pages={135–144} }
 @article{ojiambo_mibey_narla_ayiecho_2003, title={Field transmission efficiency of Alternaria sesami in sesame from infected seed}, volume={22}, url={http://europepmc.org/abstract/AGR/IND44667546}, DOI={10.1016/s0261-2194(03)00144-3}, abstractNote={Four sesame accessions with varying levels of susceptibility to Alternaria sesami were selected from an advanced germplasm collection of the Sesame Improvement Project to determine transmission efficiency of A. sesami from seed under field conditions. Accessions were sown in field plots in Siaya, Kenya, in March and October 1995 during the first and second rainy seasons, respectively. Incidence of Alternaria leaf spot (ALS) following artificial inoculation averaged 85–95% for all accessions in the two seasons. Infection of A. sesami in harvested seed averaged 11.8% for accessions SPS SIK 013, 9.4% for SPS SIK 110, 9.7% for SPS SIK 121 and 6.8% for SPS SIK 130. Three hundred seeds from the harvested seed lots for each accession were sown in disease-free field plots at Kibwezi, Kenya, in March and October 1996 for two seasons. Typical disease symptoms appeared about 5–6 weeks after sowing. Disease progress curves were better described using the Gompertz rather than the logistic model for all the accessions evaluated. Disease onset (Yo) was nearly the same in both seasons but the rate of disease increase (r) was faster in the first season than in the second season. Rate of disease progress was consistently higher for SPS SIK 013 than SPS SIK 130, in both seasons. Area under the disease progress curves for accessions was significantly higher (P<0.01) in the first season compared to the second season. Disease incidence averaged 0.0% for SPS SIK 110, 1.5% for SPS SIK 130, 3.3% for SPS SIK 121, and 3.6% for SPS SIK 013. Transmission efficiency of ALS by seed ranged from 0.0% to 40.7% and was significantly higher for SPS SIK 013 and SPS SIK 121 than for SPS SIK 110 in the two rainy seasons.}, number={9}, journal={Crop Protection}, publisher={Elsevier BV}, author={Ojiambo, P.S. and Mibey, R.K. and Narla, R.D. and Ayiecho, P.O.}, year={2003}, month={Nov}, pages={1107–1115} }
 @article{ojiambo_nyanapah_lung'aho_karinga_kidanemariam_2000, title={Comparing different epidemiological models in field evaluations of selected genotypes from Solanum tuberosum CIP population A for resistance to Phytophthora infestans (Mont.) De Bary in Kenya}, volume={111}, ISSN={["0014-2336"]}, url={http://europepmc.org/abstract/AGR/IND22049397}, DOI={10.1023/a:1003810408976}, number={3}, journal={EUPHYTICA}, author={Ojiambo, PS and Nyanapah, JO and Lung'aho, C and Karinga, JK and Kidanemariam, HM}, year={2000}, pages={211–218} }
 @article{vega-sanchez_erselius_rodriguez_bastidas_hohl_ojiambo_mukalazi_vermeulen_fry_forbes_2000, title={Host adaptation to potato and tomato within the US-1 clonal lineage of Phytophthora infestans in Uganda and Kenya.}, volume={10}, url={http://europepmc.org/abstract/AGR/IND22085340}, DOI={10.1046/j.1365-3059.2000.00487.x}, abstractNote={Twenty isolates of Phytophthora infestans from potato and twenty‐two from tomato, collected in Uganda and Kenya in 1995, were compared for dilocus allozyme genotype, mitochondrial DNA (mtDNA) haplotype, mating type and restriction fragment length polymorphism (RFLP) fingerprint using probe RG57. Based on RFLP fingerprint and mtDNA haplotype, all isolates were classified in the US−1 clonal lineage. Nonetheless, isolates from potato differed from isolates from tomato in several characteristics. Isolates from potato had the 86/100 glucose‐6‐phosphate isomerase (Gpi) genotype, while those from tomato were 100/100, which represents a variant of US−1 that had been identified previously as US−1.7. Furthermore, while pure cultures of the pathogen were acquired from infected potato leaflets by first growing the isolates on potato tuber slices, this approach failed with infected tomato tissue because the isolates grew poorly on this medium. Tomato isolates were eventually purified using a selective medium. Six isolates from each host were compared for the diameter of lesions they produced on three tomato and three potato cultivars in one or two detached‐leaf assays (four isolates from the first test were repeated in the second). On potato leaflets, isolates from potato caused larger lesions than isolates from tomato. On tomato leaflets, isolates from that host caused larger lesions than did isolates from potato, but the difference was significant in only one test. The interaction between source of inoculum (potato or tomato) and inoculated host (potato or tomato) was significant in both tests. Isolates from tomato were highly biotrophic on tomato leaflets, producing little or no necrosis during the seven days following infection, even though abundant sporulation could be seen. In contrast, isolates from potato sporulated less abundantly on tomato leaflets and produced darkly pigmented lesions that were most visible on the adaxial side of the leaflets. Nonetheless, all isolates infected and sporulated on both hosts, indicating that host adaptation is not determined by an ability to cause disease but rather by quantitative differences in pathogenic fitness. Assessment of Gpi banding patterns, mtDNA haplotype and RFLP fingerprint of 39 isolates from potato collected in Uganda and Kenya in 1997 indicated that the population had not changed on this host. The population of P. infestans from Kenya and Uganda provides an interesting model for the study of quantitative host adaptation.}, number={5}, journal={Plant pathology.}, author={Vega-Sanchez, ME and Erselius, LJ and Rodriguez, AM and Bastidas, O and Hohl and Ojiambo, PS and Mukalazi, J and Vermeulen, T and Fry, WE and Forbes, GA}, year={2000}, month={Oct}, pages={531–539} }
 @article{ojiambo_narla_ayiecho_mibey_2000, title={Infection of sesame seed by Alternaria sesami (Kawamura) Mohanty and Behera and severity of Alternaria leaf spot in Kenya}, volume={46}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0034035753&partnerID=MN8TOARS}, number={2}, journal={International Journal of Pest Management}, author={Ojiambo, P.S. and Narla, R.D. and Ayiecho, P.O. and Mibey, R.K.}, year={2000}, pages={121–124} }
 @article{ojiambo_ayiecho_narla_mibey_2000, title={Tolerance level of Alternaria sesami and the effect of seed infection on yield of sesame in Kenya.}, volume={7}, url={http://europepmc.org/abstract/AGR/IND22088533}, DOI={10.1017/s0014479700003069}, abstractNote={Field plots of sesame (Sesamum indicum) with six different levels of seed infection with Alternaria sesami were monitored for Alternaria leaf spot severity at Kibwezi, eastern Kenya. The aim of the study was to determine the effect of seed transmission of the pathogen on yield and tolerance level of the fungus in sesame seed. Increase in percentage leaf area diseased and percentage defoliation fitted the Gompertz model more closely than the logistic model. Areas under disease progress curves (AUDPC), infection and defoliation rates varied among the six infection levels. Disease severity increased with increase in seed infection and was least and most severe in plots established with seeds with 0 and 8% infection levels respectively. Yields ranged from 234.9 to 300.1 kg ha−1 compared with 312.5 kg ha−1 for the control, and losses due to seed infection ranged from 4% to 25%. Disease severity was negatively correlated with seed yield, 1000-seed weight and seeds per capsule. Alternaria leaf spot severity had a major effect on the seed weight component of yield. Tolerance level of A. sesami in sesame seed was determined to be less than 2%.}, number={3}, journal={Experimental agriculture.}, author={Ojiambo, PS and Ayiecho, PO and Narla, RD and Mibey, RK}, year={2000}, month={Jul}, pages={335–342} }
 @article{ojiambo_ayiecho_nyabundi_1999, title={Severity of Alternaria leaf spot and seed infection by Alternaria sesami (Kawamura) Mohanty and Behera, as affected by plant age of sesame (Sesamum indicum L.)}, volume={147}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0032848347&partnerID=MN8TOARS}, number={7-8}, journal={Journal of Phytopathology}, author={Ojiambo, P.S. and Ayiecho, P.O. and Nyabundi, J.O.}, year={1999}, pages={403–407} }
 @article{lung'aho_ojiambo_kidanemariam_1998, title={Yield stability analysis of promising potato clones in mid and high altitude regions of Kenya}, volume={6}, DOI={10.4314/acsj.v6i2.27809}, number={2}, journal={African Crop Science Journal}, publisher={African Journals Online (AJOL)}, author={Lung'aho, C and Ojiambo, P S and Kidanemariam, H M}, year={1998}, month={Feb} }
 @article{scherm_ngugi_ojiambo, title={Trends in theoretical plant epidemiology}, DOI={10.1007/1-4020-5020-8_5}, journal={Plant disease epidemiology: facing challenges of the 21st Century}, publisher={Springer Netherlands}, author={Scherm, H. and Ngugi, H. K. and Ojiambo, P. S.}, pages={61–73} }